Agree with Ribbon13. Get a "both" enclosure - costs about ten bucks more, but gives you great flexibility. USB has a faster transfer rate in the specs, but not in real life. Also - Firewire B is much faster.
I just figured out and did an external drive that is faster than either USB2 or Firewire - that is SATA. Yes - it is possible, and it is much faster than either USB 2 or 1394.
There is a front panel assembly that places a SATA data and Molex power port in a floppy drive space. After setting up the SATA/RAID driver and enabling it - it is also bootable, etc.
Here's an enclosure that handles all 3:
Incl
Here's the easy front panel gizmo:
SATA
Just hit the "Go" button for 202 0701