Shaw is right. The theoretical speed of USB 2.0 might be faster than FireWire, but you have to keep in mind that these are maximum theoretical speeds. Not set in stone speeds. So while USB 2.0 is certainly advertised as being faster, in real world situations FireWire outperforms USB 2.0.
That, and given the fact that DV to computer transfers takes place in real time, and that FireWire is more than capable of keeping up with the demand, the virtues of USB 2.0 are moot. The only real advantadge I see to is that USB is more 'plug and play' friendly than Firewire. But given the lack of support it has with NLEs right now, narrowing your camera choices to something with USB 2.0 is akin to shooting yourself in the foot.