Just to be clear, I would rather NOT use the term "svhs". This is further confusing the issue, since svhs recording does not in any way enter into the equation here. It's "S-video" (NOT svhs) vs. Firewire.
Now, as far as PQ quality goes, I don't _think_ you will notice any difference in PQ between the 2. In BOTH cases there is a re-encoding that has to occur to get the data onto DVD. In both cases, the SAME MPEG encoder (the one in the DVD recorder) will be used. Since I also assume that the MPEG encoding step is the biggest source of PQ degradation, (much bigger than d/a-a/d conversion, or DV decode steps) the PQ should therefore be comparable.
Keep in mind that I have never actually done Firewire transfer to DVD. My E80 doesn't have it. This is all based on conjecture. I haven't even tried S-video, but I _have_ transferred some miniDV material to E80 using composite, and even that produced very good results. I would expect even better with S-video. Anyone with practical Firewire vs. S-video, please chip in!
As far as results using a PC, I can't say much, only conjectures again. This would be very similar to using Firewire on a recorder. The DV data will be transfered to the PC as a lossless digital copy. But instead of the recorder doing the conversion to MPEG, your PC's DVD authoring software would do it. So depending on the quality of the MPEG encoder on your PC compared to the one in the DVD recorder, the results may vary..
Oh, and to answer a previous question, there is NO WAY to get a lossless transfer from miniDV to DVD-R, not even with Firewire. (of course you can use a PC to copy the raw DV data to DVD-R media, but that doesn't count, since you will NOT be able to play it on a DVD player.) As mentioned before, the formats are different (DV vs. MPEG)