Depends on how much rendering and back and forth you aniticipate over Firewire. DV is compressed and every time you render anything, the file gets decompressed then recompressed. You can come in FireWire and once decompressed, capture as 1:1. Do all your work as 1:1 till the final output back to DV over FireWire or encode to MPEG2 for DVD, etc. That would be a higher quality than capturing 1:1 from analog component.
If the question is work in DV25 over FireWire versus analog 1:1, it would be pretty close and most likely not noticeable. It depends as the previous poster noted, how good the A to D converter is as well as control over the color and levels when capturing.
Michael
anything 24fps