PS/2 isn't as reliant on system resources so they're far more effective on non-quite-up-to-date machines. If you have windows XP or a program called PS2rate you can bump the response rate of your PS/2 mouse upto 200hz. The standard PS/2 response rate is a meager 80hz
USB mice can't get past 150hz
On the other hand though USB is hot swappable meaning you can plug it straight in and not need to reboot the PC like you would a PS/2. To be honest for gaming there is no better way both will give you good results if you try them both out I would highly doubt you would notice a difference at all. PS/2 is on its way out if you ask me a lot of motherboards are now legacy free and dont carry PS/2 ports, and when was the last time you saw an up to date mouse that nativly used PS/2? Its all USB with PS/2 adapters just incase you need it.
Dont get me wrong theres nothing wrong with PS/2 mice, if you own one dont go out and buy a new mouse just because its USB, because at the moment theres no real point. At the end of the day its just connectivity.