Actually, the two are very closely matched on throughput if you're comparing USB 2.0 and if you're looking strictly at max bit rate. Firewire is spec'd at 400MB/Sec, and USB 2.0 at 480MB/Sec. The primary difference is that the Firewire interface on your computer has hardware to handle the process of assembling the stream of bits from the external device into a collection of bytes in memory and to handle the overhead of the communications protocol, while the USB requires the computer's processor to do that. Thus if you are using a lot of VST filters and effects for processing your audio as you record, or using software synthesizers to do instrument tracks along with the audio you're taking in through the interface, the total CPU load will start pushing the limits of the machine sooner with USB.
For just a single channel or a stereo pair, recording to the hard disk without a lot of signal processing you probably won't see any difference, and the USB interfaces are a bit cheaper and there are more to choose from. If you're really going to load up your machine with audio bells and whistles, though, or you're looking for a 12- or 16-track IO setup for real multi-track recording, you'll really benefit from the Firewire.
Good points, but really in laymen's terms... Don't go USB. All your peripherals (mac and PC) are on your USB ports (even your mouse and keyboard). You want to squeeze every bit of bandwidth out of your chosen interface as possible, so here are some suggestions:
1) On most MBs, (even the Mac Pro Desktop -- some versions, not all they keep changing their MoBos), the firewire port and the USB ports are running off the same bus... BAAAAD.
Get a Firwire card Which one? If you're on a PC... just any PCMCIA firewire card will work.
2) Go Firewire for your breakout box. m-audio, motu, and several others make great boxes.
3) Don't put ANYTHING ELSE on that bus. Any removeable HDDs should go on USB, and NOT the firewire.
4) When you're ready to record... unplug any unnecessary peripherals, and turn off any non-essential memory resident programs. Here's how to make sure on windows:
Start->Run
type msconfig
click deselect all
(now go through before clicking ok, and reselect anything having to do with cakewalk or your breakout box... and that's ALL)
click ok, and the computer will reboot, now you'll get a warning when the computer comes back up... just check the "don't warn me again" box.
Now nothing other than OS, cakewalk, and your breakout box is running, and your computer should perform quite well.
When you're recording multiple tracks simultaneously, performance is key!!!