Get one with both midi and usb ports. That way if usb lags on medium/high cpu load you can see if it'll run better when connected using the midi ports.

This was the case with my mpd-16 which responded unreliably when there was a lot going on in a track. When using midi out, the timing was a lot more stable.

It may have just been crappy drivers or something... and it definately wasn't my computer becaus it happend to a lot of people.

Just throwing in some more experiences in to confuse you :)