Actually, one reason for using USB...I had a MS Explorer Optical that died on me while plugged into PS/2 and would come on and off (loose cables inside the mouse). The problem seems obvious to me now, but when it was happeneing, for the life of me, I couldn't figure out why my comp would freeze randomly. I ended up buying a new motherboard, frying a vid card (replaced by the nice folks at Creative), and almost a new processor until I magically discovered that the dying mouse have been causeing the lockups. Pain in the ass....with USB, it's hotswappable and if it breaks on you, no biggie.
However, if you only have limited USB ports, then get the regular ones. Not likely that a mouse dies on you.