I have a Geforce 2 GTS and I can at least say this much.
The anti-aliasing on GTS is only so/so. In OpenGL I tested Q3A. I put it to 2x2 AA mode.
At 1024x768x32 there was a BIG performance hit. I don't know Fps difference but it went from slippery smooth to jerky and irritating. The thing that really bugged me was that if I looked around for a second I could still see some minor jaggies here and there (although there definatly was an improvement in Image quality)
At 640x480x32 I compared AA on and off and I could see that there was a large improvement in Image Quality. And at 640x480x32...performance was not a problem..but I think the higher resolution w/out FSAA looked better anyways so hmm?
The D3D controls give the choice of up to 4x4 AA mode but I haven't tried those out....yet..
I guess the Voodoo 5 is supposed to be faster and look better compared to GTS WHEN FSAA is ON. Although I have no V5 to check it out..I'd bet that it's true. I guess FSAA was included on Geforce cards kinda prematurely (nVidia fearing 3dfx?). But I guess it's also a good thing. Imagine if they left 32-bit 3D color out of the TNT just because there was a performance hit?
Well anyways I guess if you're into FSAA then V5 would probably be better.
If no FSAA then GTS.
The V5 has a better looking FSAA, but the GTS can run at faster framerates at much higher resolutions than the V5-- which in most people's opinion practically negates the need for the FSAA in the first place.
In other words, the choice is six of one, or a half-dozen of the other.