Some (all?) DV camcorders transfer the raw DV data over Firewire but recompress the video when tranferring over USB. DV camcorders are really meant to be used with Firewire.

FWIW, I like IEEE 1394a because it seems to have a better sustained transfer rate over USB 2.0, and I've had some bad experiences with USB 2.0 enclosures that I've never had with any of my 1394a ...