If you are recording directly to the externak disk then I'd say go for FireWire, or better yet, if you can find/afford one, UltraWide SCSI. USB 2.0, although comparable in throughput to FireWire on paper, is still ill suited for sustained high-speed transfers thus you would get less # of simultaneous tracks on playback and recording.
OTOH if you are simply copying from an internal hard drive to an external one for backup or transfers, then USB should be fine.