If you are simply recording/playing back a couple of tracks at a time, it's unlikely to
make much difference for your needs.
In general, FW offers far better
performance than USB when moving large amounts of streaming data around - you will be able
to get more simultaneous streams of I/O, or more disk tracks (in the case of FW hard
drives) with less CPU overhead than using USB.
(USB requires the computers CPU
to regulate transfers, whereas as FW has dedicated chips to do much of the gruntwork.
Also, USB is really optimised for the transmission of smaller data packets, whereas FW was
designed from the ground up to transfer/stream large amounts of data.)
audio use, where we are usually dealing with larger amounts of data, this means that
generally speaking, FW *is* better than USB 1.0/2.0. Leave USB to mouses, scanners,
dongles etc, and use FW for the data. Personally, I avoid USB audio interfaces and would
never buy one unless there was really no other choice.
But like I say, for
extremely light use, USB will largely be fine for you.