There isn't really any performance difference, but each has their pros and cons.
PS/2 is a legacy port, which means if the computer is on, you need to restart the computer for it to be detected. It's not hot swappable, which means unplugging it and replugging it won't have it work again. The good thing is that you don't need drivers, as legacy peripherals are natively supported by Windows.
USB isn't legacy, which means that you can plug it in and unplug it whenever (i.e. hot swappable). For some of the older USB peripherals, you'll need drivers, but now that USB has become mainstream, Windows natively supports it without the need for drivers.
My opinion is that if you can, go with USB peripherals. Although there isn't any real performance difference and PS/2 is as reliable as ever, USB has a potential future, unlike PS/2 where it's destined to the graveyard like AGP and other computer technologies.