USB2 is spec'd at a higher speed than firewire (480 mbps vs 400) but neither deliver at their upper end. I don't know of software to capture DV via USB2 but it certainly may exist - perhaps it came with your dazzle product? If you can capture this way the quality shouldn't differ from a firewire capture, although I don't know what codec you would end up with for editing. Could be Microsoft DV AVI I suppose, which would be fine. I don't know what settings are available for MPEG-2 encoding via the dazzle. Probably best to run a test with a minute or two of video, using both and making your own judgement.
David Hurdon