They're both similar in execution and used as counters against hip techniques. Are there any reasons to prefer using one instead of the other in various situations?
I can’t comment, so I'll answer it here instead.
As @Dave Liepmann said, it depends on your strengh and your ability. Utsuri Goshi is hard to do. The hardest move I've done, seriously.
Using the “right” move is always a decision based on your opponent's reaction.
If you start and manage to raise him off his feet (which is the first step of both moves) as set-up for ushiro goshi, 99% of the competitors will use their right leg to "hook" the inside of yours to prevent you from shifting correctly and easily over your shoulder/to your side, and often your grip isn't ideal and they will slip, turn, and try/manage to fall on you if you still try to get the move off.
I was a huge Harai goshi user, and got ushiro goshi used a lot against me... And you train yourself a lot to counter it. It's obvious when someone tries it, and not too hard to counter. I'd say in 14 or 15 years of competitions, it has been successfully used less than 10 times against me, and threw that about 2 or 3 times for anything higher than yuko.
On the other side, Utsuri ghosi gives you more control over your opponent, since once you manage to raise him off his feet, his first reflex will be to press or push against you, thus making it easier to pass in front of him, and once you do, you cannot miss scoring Ippon. Plus it's a hugely spectacular move to pull off.
But most of all, it's counterintuitive for someone to really block you correctly, and it has a big surprise factor. I was training with the elite team here, and I've never seen anyone practice how to counter or dodge utsuri goshi, because it is seriously hard to do.
I don't have an utsuri goshi, but I like ushiro goshi when my opponent hasn't broken my balance forward. After a successful hip block, ushiro goshi often feels like just an extra little "pop" of my hips to send someone up.
My understanding is that utsuri goshi is a somewhat more difficult and skillful throw, but it seems to afford a greater deal of control over the person being thrown. That is, uke can be controlled longer during utsuri goshi than with ushiro goshi, since uke can be carried on the hip and connected to your center further along the trip to the ground. Then again, like I said, I don't use the throw.