The difference isn't quality, but interface speed. I don't have the specs, but firewire is substantially faster than the latest USB standard. When transferring video, data throughput speed translates to video quality. I don't think DV can be transferred in real time via USB because USB is too slow. That said, you can transfer a DV file from, for example, your PC to a USB connected hard disk with no loss of quality. It just won't be in real time.
