Hi Ronin,
If you are just going to be recording a few tracks of audio, then a USB interface will probably fill your needs just fine. I recorded for a couple years with a Tascam US-428 and never ran out of power with it. I'd build up songs with 20-30 tracks, but never needed to record more than 4 tracks at once, so it worked fine.
I now have a firewire interface and can record up to 18 tracks at once, if needed. The most I've needed so far was 6- but I still couldn't have made the session with the USB device. I have other plans, as well, that will eat up 16 tracks at a time, so it was worth the extra money for the firewire.
If you don't need the extra bandwidth offered by firewire, why pay for it? If you don't see yourself recording more than 4 tracks at once for a while, save the money and go with a USB interface.
Take care,
Chris