I prefer PS/2 for mice, mainly because peripherals are all going USB these days, and I don't feel like buying a hub to be able to run my mouse, joystick, wheel, scanner, and printer.
as for ball vs optical, I have yet to find a new mouse that's as comfortable as my old Logitech Mouseman Wheel - until I do, I'm stickin' with my old ball mouse no matter how much i might want optical
