USB is faster and it is better when there is nothing saturating the USB with traffic and CPU can keep up. Most of the time there will be no noticeable difference using the USB mouse or PS/2 mouse. But if CPU is busy and USB bus is saturated, USB mouse will start to be less responsive than PS/2 mouse. In the extreme cases even USB keyboard will become unresponsive where the old PS/2 keyboard still works fine. This can make a difference between being able to terminate a misbehaving process and resume working or having to power-cycle the computer.