it comes with ps2 because there's still people that buy those mice that have no usb. i would go with usb, because unlike ps2 you can take it out whenever you want without turning the computer off. if you do use ps2, use ps2rate to make it as smooth as the usb connection.
That confuses quite a few people, as necro says, the usb is the newer and better, so go with that
There is no advantage to USB. I have a PS/2 mouse and I can pick out single pixels on a .22 dot pitch monitor with it. Your PS/2 port is there, you might as well use it. Also, I am not sure if all games support USB peripherals.
In many instances, it is proved that there is no NOTICEABLE differences between USB and PS/2. THose numbers are useless if u can't even notice it. Go with USB if u frequently switch your peripherals, and if it is easier to route the wires. But use ps/2 if you don't even remove your mouse, y waste the ps/2 and occupy a usb for a "smooth and precise connection" that you can't even notice.