Do you use AA? 9600gt without any doubt. Otherwise, it's more debatable. If your res is 16x12 or less the performance is close (with AA off), above that the 9600gt is noticably ahead.
Error, the gt actually has the faster gpu, gs has 32(?) additional shader units. The problem is that the shader units would allow you to increase certain detail levels, but increasing those detail levels also increases the necessary vram and memory bandwidth... so you've got an advantage in crunching that the card doesn't have the muscle or pipe width to really take advantage of. Current games seem more dependent on Vram <512meg and Mem archi <256bit (above these numbers seems to only show gains at very high resolutions, beyond what most people run) than on shader units above 64. So you run into what you see in many benches. At very low settings all the cards from the 88, 96, and 98 families kind of pile together... then the 8800gs falls behind but the 9600 stays with the pack, then it falls behind, then the 8800gt and so on until you're at the highest reso's and there's only the Ultra and/or GX2 left. So shopping for Gfx cards right now really depends on what games, what settings and what reso's you want to run. There's no one "best card" for everyone. If you're buying for the future, the 8800gt is probably the safest bet for a cheap card. 9600gt and 8800gs both have limitations that could cause large performance gaps in future games... but buying for the future is really a big gamble... usually the best thing to do in that case is just buy the most card you can afford and hope it ages well. I say buy for today and start saving for the future.