I agree, USB. I'm using my grandparent's optical mouse right now on PS/2 and it's response and refresh just aren't as good as when I use the exact same mouse on my computer with USB.
In fact, I use a linksys KVM between an M$ and a Linux box, but don't use the mouse port because then I'd be forced to use PS/2. I previously thought the lag was because of the KVM, but discovered it was the PS/2.