Also, if you're adamant about getting the absolute highest performance from your computer, whether it be for games, scientific simulation, or whatever, PS/2 is a better choice due to the interrupt based nature of the protocol, as opposed to the polling based USB protocol.
USB really is a horrible horrible interface. It's chatty, it's polled, and the spec is absurdly over complicated. But it's the classic case of "good enough". Because it works for most things most of the time, it's the current standard.
There are much better standards for different purposes, but USB is good enough for most everything. And it's CHEAP. I read somewhere recently that PS/2 was dropped not because it wasn't needed anymore, but because it was actually more expensive to include than another pair of USB ports. That should tell you something right there...