USB is plug and play, ps2 is not. If you are putting it in a cabinet and will never move/unplug it, then the standard ps2 is the only thing you need. I use the usb option on my desktop cp so I can just reach down and plug into the front of the pc when i want to use it. (for some reason, the wife doesn't like that thing sitting on the computer desk all the time.

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Oh yeah, the cable is ps2 on one end, and usb on the other. I believe it is special just for use with the ipac.