This question came up to my head but just wanted to ask this question. The only difference is that Volt Switch is special whereas U-Turn is Physical. This is for Sw/Sh competitve but you can also add other games as well.
Volt switch can be blocked, U turn can't. It is that simple. The point of those moves is to be able to remain a step ahead of your opponent as your Pokemon scares out theirs and if it's blocked then you lose that advantage