USB definetly.
There are much niftier things you can do with the device once it's hooked up that way too... like an editor... quick upload/download of presets etc....

Midi doesn't do anywhere NEAR enough data to warrant needing to worry about USB2.

Midi is essentially serial... that's why you can fit 8 inputs and 8 outputs into one device such as the MOTU midi xpress usb... all with one usb cable ;)