FireWire transmits the HDV recorded on your tape, and is the way to transfer taped recordings. No quality difference from the original footage (unless you set your camera to output DV instead of HDV via FireWire). I am reasonably sure about this, but not 100%. I admit that the technology is still rather new to me, and I haven't worked with it much.
This thread should clear things up a bit:
http://www.hv20.com/showthread.php?p=572…
If you capture to your computer via HDMI, DIRECTLY from the CAMERA (i.e. live, not taped), the quality will be better. Once it's on the tape, though, it's HDV, and that's the same whether FireWire or HDMI.
Of course, in order to do that, you'd need a way to get HDMI IN on your comptuer. That thread explains.
Thanks
Kyal Mills