I wonder the same regarding why they made it PS2, as for PS2 vs USB... PS2 gives you 200hz poll rate while USB 125hz (unless product specifies other value, mouses come with 500+ nowadays), although PS2 is a little faster vs generic USB most likely these products are designed to work at a faster poll rate so go ahead with USB.
See if the problem doesn't raise up again, so the problem on the XAI also disappeared? could be that your PS2 port is having some issues, not a big deal since it's practically a useless port nowadays.