Well as cut as dry as I can state this...Based on pure sound alone, the USB M-audio stuff
(fast-track-pro...and the like) is usually 150$ to 200$. The Pre-sonus Firebox is 300$ because its firewire and has "better" pre amps. Anyone who has experience with these, as they are pretty popular.. is there that big of a difference in the quality of the pre amp, and in result, your sound? Say your using a 200$-400$ condensor microphone. Is it worth the extra 100$?
Mortalis: True Massachusettsian Thrash Metal \m/

Hypocrisy-"Hell, the ****ing corn on my dinner plate is heavier than Korn."
you wont notice much difference in preamps until you get past the $600 mark. Im sure you would not hear much difference at all between the two interfaces.
The only point to firewire is to transfer more tracks at one time without mixing. It doesnt change the sound.

so yea, stick with the M-Audio unless you want to track more than two inputs at one time.
The presonus preamps kinda suck. I have to use the artificial boost provided by in the control panel settings, they suck so bad that the signal has to have maximizer treatment just to be decently audible and that brings in other problems as well. But as an interface, it's great. I love it.