Firewire is much faster than USB, but depending on your needs, the extra speed may be a waste of money.
Consider your needs:
1) how many simulataneous tracks do I want to record?
2) how many simutalneous tracks do I want to play back?
3) what sort of growth potential do I want in my system (i.e., what will I be doing a year from now...)
etc.
Compare your needs to the Edirol specs and a make a choice.