PS2, as far as I know, can only take 3 commands at once. So in that regards, USB would be better.
As far as a gaming keyboard goes, the best I've ever played on? Dell Quiet Key keyboards.
http://store.yahoo.com/directron/dell104.html at $3, you can't really go wrong. You can also find them on ebay for cheap. They are genuinely some of the most comfortable, easy to use keyboards I've ever used, and they are GREAT for gaming. If a USB version exists (I actually don't know), all the better.