Basicly.. it boils down to which one will you use more.
USB is more common in the PC industry whereas Firewire is known in the Mac World.
Once you've choosen which to go with, then you look at speeds.
If you have a PC, i say go with USB since its more common.
Firewire is fast and great, but unless your only working with Macs, i'd suggest USB
You can get Firewire cards for PCs too.. but if you ever goto a friends house with only USB... your pretty much screwed.