The advantage of Firewire is a higher sustained bandwidth. Most USB interfaces will max out at 24 bit 48KHz, whereas FW will step you up to 96KHz. FW interfaces more easily support multitrack recording (my NRV10 handily routes 8 channels, and most control surfaces are firewire), and my FW410 tackles 8 outs for some pretty flexible routing. USB has no problem handling stereo though, and I believe the Fast Track Pro has 4 outs to play with if you want to A/B monitors. Firewire can also sometimes be daisy chained. My FW410 and NRV10 (both M-Audio) share a driver, and show up as one giant recording interface.
The question is, do you need any of that? I've never had any issues with latencies on USB1.1 cards. 24 bit 48KHz recording is really decent quality. USB tends to have more universal power and driver support, and some FW cards can be really flaky about hotswapping (I had a client fry the FW port on his Powerbook by trying to unplug a Firewire Solo while his mac was on).
Whats more important than the cable used is looking up specs on the card.
What's the signal to noise ratio? Frequency response? Available gain? Does it have the ins and outs that you need?
USB2 is technically faster than firewire (480Mbps compared to 400Mbps), but it seems to be more of a burst protocol. In testing (check out old Tomshardware posts for more info) firewire tends to have a higher sustained rate though.
Will this mean anything for soundcards? No not really.
Most cards tend to be USB1.1 (MBox2, Fastrack Pro) though so check your specs!