Neither. The best solution is a single 8800 GTX, maybe factory overclocked. Example: BFG 8800 GTX OC2, or eVGA 8800 GTX ACS3.
You can compare a single card with two cards, but it's a bit tricky:
1. go tohttp://www23.tomshardware.com/graphics_2007.html?modelx=33&model1=853&model2=706&chart=298
2. write down the fps (30.5 for Ultra, 28.7 for non-overclocked GTX). By the way, this means overclocked GTX would get about the same fps as the Ultra, but it's cheaper
3. go tohttp://www23.tomshardware.com/graphics_sli2007.html?modelx=33&model1=805&model2=807&chart=358
and write them down again: 38.5 fps or 38.8 fps for SLI 8800 GTS.
Repeat for a different game, just make sure you pick the same benchmark for both URLs. Do it only for 1920x1200. At lower resolutions SLI is not necessary.
This particular comparison shows that 8800 GTS SLI does better than a single 8800 Ultra or GTX. However, some games don't support SLI, so in those games your SLI combo would be weaker than a single GTX. Also, with SLI you pay more for electricity, there's more heat, more RAM consumed by drivers, etc. The simple rule is to do SLI only with the latest and greatest cards (8800 GTX or 8800 Ultra), never with weaker cards (including 8800 GTS).