A Katana is a single bladed two handed curved sword originating from Japan. A Longsword or Bastard Sword is a double bladed two handed straight sword originating from various European countries.
Many of the static stances used for the two weapons appear similar at least on a superficial level. Can anyone here comment on the actual differences in learning Longsword from ancient manuals and the living styles still practiced in Japan such as Kendo or Tenshi Shoden Katori Shinto Ryu?
To answer your question straight and simple: yes, they are traditionally used differently.
But to answer it with a bit more detail: yes, but it depends on a lot of things not necessarily related to the fact that you are comparing katanas and longswords per se.
Keep in mind that there are many traditions of Japanese swordsmanship (including ones with historical records) with different philosophies and goals as well as many historically recorded traditions of European longsword. To get a more accurate answer, you need to specify which traditions of both you are comparing. It's very difficult to put this simply, but there is a surprising amount of variety even within the respective traditions for the two weapons even if their stances on the surface look similar to each other. Just to illustrate the point, here's two images of a small sampling of stances from two different European longsword traditions:
Fiore dei Liberi:
Paulus Hector Mair:
Notice how some stances that are superficially similar to one another have slight differences. Porta di Ferro Mezana while superficially identical to Alber, has some differences like how retracted Fiore likes to hold that guard vs. how Mair likes to lean forward and put it out in front. In addition, Fiore treats Porta di Ferro Mezana as a variant of the Iron Gate stance which includes an alternate variant called Tutta Porta di Ferro while Mair categorizes Alber as a completely separate stance from the rest in his system. In addition, some stances are missing from each others' systems like how Fiore doesn't technically have a guard identical to the Vom Tag in Mair's longsword and how Mair doesn't have a guard like the two Posta di Donna's in Fiore's longsword. These differences are important since they highlight a different mindset to using the stances between these two schools of longsword.
Now lets look at two different traditions of Japanese swordsmanship's kamae:
Kendo Kamae:
Katori Shinto Ryu Kamae:
Notice again that while some stances are superficially similar and even share the same names, there are differences. The feet in Kendo's Jodan no Kamae are much closer together with the back heel lifted and the body facing forward vs. Katori Shinto Ryu's Jodan which has the body more turned sideways and the feet wide apart but firmly planted. Again, some stances are missing in each others' systems like no Tori kamae variants in Kendo and no edge up variant of Waki Kamae in Katori Shinto Ryu and other kamae that are kind of similar are shuffled around or named differently. Again, these differences are a result of differences in mindset regarding how the sword is to be used in these different schools of Japanese swordsmanship.
It may be tempting to look at stances and superficially draw similarities between them and assume they are used in the same way:
But you have to keep in mind that these are static comparisons while swordsmanship is an activity that is in motion. It's very difficult to give generalities about how using a longsword is different vs. using a katana without more specifics about which styles you are comparing since there's a staggering amount of variety of subtle differences even within the same weapon type.
With that said, if we must talk generalities, we will have to look at the weapons themselves and point at the differences we can observe between them and assume these differences will lead to differences in use. In theory, just by pure physical differences in the weapons, there are some things that you can fundamentally do with a katana that you cannot with a longsword and vice versa.
GRIPS
For example, the "German grip" as seen in the Lichtenauer tradition and shown by Skallagrim here is generally impossible to do with a katana since traditionally katanas have a tsuba/disc guard that gets in the way of such a grip.
However, I must caveat that not all traditions of longsword use the German grip (e.g. Fiore and Italian traditions don't seem to use it as often) and there are some longsword designs that wouldn't allow an easy German grip:
Plus there are niche styles of katana that could theoretically allow a thumb grip:
And there are some unconventional, but similar grips shown in Japanese artwork depicting swords that involve fingering the blade in some way:
BLADE DESIGN
Another difference that seems apparent is the number of edges of the swords and blade design. The katana is curved and the longsword is straight which to some may indicate that a katana is a better cutter and the longsword is a better thruster and that you should optimally use one type of attack more on one than the other. There is some truth to this, but realistically both the katana and longsword are cut-and-thrust swords and traditions from both show both cuts and thrusts being used and the amount of what attack to use will depend on the tradition:
A longsword cutting:
A katana thrusting:
With that said, the curvature on a katana does somewhat off-put the angle at which thrusts can be performed vs. the straighter blade of the longsword which is more intuitive to thrust with. In addition, most longsword designs tend to have highly tapering blades which can be used to pierce mail armor more easily.
Contrast that with katana blades which are generally quite thick and stiff blades with wide hatchet points which are good for cutting. Plus, the curvature does aid in extracting the blade from a target more easily during a cut.
The curve in addition to aiding cuts can sort of act like a weird shield since the curvature covers a wider line of attack when compared to the linear blade of the longsword when trying to parry a cut or can even act to passively displace a blade sideways when thrusting.
It has to be mentioned though that not all katanas are curved enough to have the above benefits/disadvantages of curvature since some can be almost straight:
And the katana's ancient predecessor, the chokuto (which in Japanese means straight katana) was straight:
Plus, there exists some longsword designs with almost no thrusting capacity whatsoever and are designed to purely be cutters.
And there are katana blades which are clearly designed to be pointy thrusters.
EDGES
The double-edged blade of the longsword allows false edge cuts to be performed by flicking the hand upward vs. a single-edged blade which can only cut with the true edge and requires flipping the sword around to perform similar rising cuts. Certain techniques however that involve touching the blade may be more risky to perform with a double-edged sword. Iaijutsu/Iaido are a bit more risky to do with a conventional longsword since the two edges could lead to cutting oneself if done in the traditional fashion shown in iai here since it involves placing the spine of the blade against the hand and sliding it across the skin.
With that said though, yes people did historically grip sharp longsword blades with their bare hands and it could be possible to perform some blade-touching techniques like iai if you're very careful:
And, there were double-edge katanas historically:
And single-edge "longswords" (if you play a little loose with the semantics of what a "longsword" is and count langesmesser and kriegsmesser as "longswords"):
And people have come up with ways of sheathing double-edged katanas in modern practice
WEARING METHOD
Another difference some may state is that the way that the katana is worn traditionally (i.e. edge up in an obi) does provide some benefits vs how a longsword is worn since the obi does hold the sword very securely and comfortably to your core and allows easy manipulation of the scabbard through the belt for iaijutsu/iaido vs. a longsword which traditionally is worn from a belted suspension system:
With that said, the katana's predecessor and look-alike, the tachi, was worn like a longsword:
In addition, it's not impossible for a person to quick-draw a longsword like a katana with a suspension system.
Plus there's nothing stopping you from sticking a longsword in an obi as long as the fittings don't get in the way of saya-biki, it just wouldn't be traditional.
Another thing to consider regarding obi wear vs. suspension wear is that it is generally easier to carry larger swords from a suspension system since there is more space available to draw a longer blade since the sword hangs beneath the waist while obi wear tends to put the sword at or above the waist. However, it must be mentioned you can still draw and use really large swords even if wearing a sword from the obi.
GUARD DESIGN
One other seemingly obvious difference is that most longswords have a large crossguard or variant of that which in theory provides more hand protection. In actuality though you need to actively intercept cuts with a crossguard by positioning the sword so that the guard gets in the way of a cut and denies the line of attack to the fingers which is something that you don't need to do as much if you have a round disc guard above the hand since it passively covers the top of the hand pretty evenly all around. Really though, relying on the crossguard, let alone any guard, by itself can be quite unreliable hand protection without addition of side-rings or other extensive furniture on your guard which means your fingers are still quite vulnerable to being hit with a traditional longsword. That's why HEMA longsworders wear these in competitions and don't just rely on the crossguard to protect themselves from cuts:
If you ignore "traditional longswords" though you do get swords with generally more hand protection than katanas which allow more point forward guards without presenting as much of a risk to your hand getting hit.
However, historical Japanese disc guards can reach rather large sizes that would be more than sufficient to protect the hands; up to 4 inches in diameter or more in some examples and there are historical examples of katana/tachi with guard extensions that could potentially protect the hand sort of like a crossguard:
Plus there are longswords with minimal disc-shaped guards like katanas:
With all that said though, it should be obvious that all these examples of niche longswords and katanas are NOT "traditional" and if you are mainly focusing on the "traditional" forms of both these weapons, it will ultimately depend on what traditions of longsword or katana in question you are comparing. But I hope the above ramble has highlighted some generalities of how the two are used differently depending on their "iconic" forms and how "non-traditional" versions of those respective weapons could lead to both swords being used in more or less a similar manner at times!
I won't pretend expertise, but I have some HEMA experience, admittedly in parallel to SCA fighting, which has some different techniques due to different rules. My Kenjutsu credentials are slightlier shabbier, in that I did my training in informal classes at work, admittedly from someone who had proper accreditation with the local schools. I actually found my prior experience was a bit of a hindrance because of the difference in styles. I'll provide my input, in advance of more experienced people offering help.
Other than the length, being edged weapons, and generally being wielded with two hands, the swords have several differences.
The first two qualities of the katana basically come down to one major issue, that Japan did not have access to high-quality iron. As a result, they used a folding process to create the katana, which resulted in a better quality of steel compared to what they started with, but still relatively brittle. Thus, the blade was blunt on side, to reinforce it, and the blade was curved to distribute the blow over a longer amount of time.
Because the katana is more fragile, you really don't so much swing it into your target like a bat so much as that you make contact, and then you move the sword to slice your target. It takes a lot of technique to do this properly, and at higher levels, especially against a stationary target, it does look like a chop because you're doing one fluid movement where the slicing is done in conjunction with the contact.
This kind of goes hand-in-hand with the armor situation. Again, there was a lack of good iron in Japan, which meant that armor moved towards a lamellar design of multiple laced scales, often of leather, rather than the broader plates of metal favored by European medieval warriors. Part of European swordwork involved heavier blows because it was the only way to damage someone through heavy plate, striking them hard enough to dent the plate or to cause internal trauma.
Admittedly, this is also something done in European longsword when you have gauntlets and vambraces, but it is more common in Kenjutsu for some techniques to use a free hand or forearm to brace your blade or to move it in a situation where you've become into too close of quarters. It does also mean that you also have to consider which direction you're cutting with.
Again, down to fragility and differences of weight, there is a stronger emphasis on deflection with the katana rather than blocking with the edge (an exception is made here, of course, where you can arrange to block an unarmed strike with your edge). With the longsword, I was taught to block with the edge because the cross-section of the sword was strongest in that direction. With the katana, I was told a direct block would significantly chip, and weaken, the blade.
As more aspects come to mind, I will add them.
One key is the shape of the blades. A curved blade naturally slides more across a target, making it easier to slice. A straight sword doesn't stay in contact as long unless you draw it across the surface. Naturally, then, the cutting technique has to be quite different. Straight swords make thrusting easier, while curved swords make cutting easier. The katana is less curved than some other cutting swords, perhaps to give more balance between cutting and thrusting, but it's more for cutting. The European swords were generally more for thrusting.
The stances might look similar because weapons in general have similar ways of wielding. For example, standing with your weapon in front of you, pointed at your opponents eyes, makes sense for many weapons, because it a) puts something between you and your opponent, and b) makes it harder to judge the exact length when it's pointed right at their eyes. It works for staves as well as blades.
