This is a good video resource from Damien Trainor after research: (feel free to add more answers)
- Straight kicks are less noticeable and less telegraphed
- Round kicks may have more power and allow to kick "wide to get past someone’s defense
Both kicks start at the same time, but the key differences to notice
are, the angle of the knee as it starts and which kick strikes first.
Even though it’s called a roundhouse kick it’s important to
remember when throwing it that you don’t want to throw it too round.
By doing this you’re telegraphing what you’re doing, it takes too long
to get to the target and you’re losing power. Think of a
circle, you want to get from one side to other as quick as possible,
fastest way to do that that is straight through the middle. Same
applies for the kick, your want the kick go at a relatively straight
angle and then turn your hip over at the last second. Key
points when throwing the kick are pivot the standing foot, throw the
leg direct to the target, turn the hip and shoulder over, guard, make
contact with your shin. Just remember EVERY technique has a
textbook way of throwing it.... however, fighting can be
unpredictable. So with that in mind, there will be times when you have
to adapt to the situation at hand in order to land something. In other
words at some point you may have to swing wide to get past someone’s
defence. That adaptation comes from experience....someone left a
comment not too long back which I thought was great “learn the rules
well before you break them”.