I haven't noticed a difference between USB and PS/2 mice, but irseg is right -- some PC's won't even turn on without a PS/2 mouse plugged in to them.
The more important question to me is whether optical mice or ball mice are better in which I can answer very confidently: ball is better.
I can't stand playing games with optical mice...it may be smoother in general usage, but in games, the absolutely control you have over the ball does it for me. I'd use optical, but it's almost too sensitive. You can pick the mouse up like .5 CM off the pad and it still works...that's bad for gaming due to the fact that precice movements are harder to make with a mouse that works even off of the pad.
-Lasereth