I just got around to looking at the comparison images in the OP. The composite shots seem far worse than what I see on my Sony CRT using composite from a Super NES. Yes, I can see very slight rainbowing on thin white lines, but no where near as bad as in the comparison shots. The Earthbound scene in particular is much worse than what I see here.

I suspect the comb filter in the video capture card is rather poor. I am not discounting the benefit of s-video, but the comparison shots in the OP are much worse than what a regular person will see with a Sony Wega CRT.

I still use composite for my Super NES. I'd use s-video, but my TV only has a single s-video input, and it is in use by my PSone. I'd get a switch box, but they all seem to have flaws due to cheap build quality (crosstalk, etc). I want a switch box that is literally a switch box, as in an A/B switch with physical connections changing inside the box. Not just a passive electronic box that changes the signal path in the circuitry. I see some mentions in this thread of a "slider" switch and that sounds in line with what I want.

Any suggestions for a true A/B switch box with s-video support?