My understanding is that the edge FW400 has over USB2.0 is that FW technology allows peripherals to communicate directly, rather than through the processor -- so when copying a file from your on-board HD to your external HD, FW allows the two drives to talk directly, while USB requires the processor to transfer. This gives a higher *average* through-put even if theoretical peak through-put is the same for the two (or even, I think, a bit higher for USB2.0).
Besides, FireWire is a far cooler name 😎, while "USB" is just some dopey acronym.😀