the GS is capable of scoring 5000 - 5500 in 3dmark 05, GT is capable of getting ~7000 i think. The numbers speak for themselves. Yes the GT is a "better performer" than a GS. The question is do you want to spend the extra money.
I have a GS....at the time of purchase they were about 33% cheaper than a GT, and i read that it was a solid performer.
I get 60 - >100 fps in hl consistently and i played thru FEAR (the game was lame...) no problem at pretty much max settings.
If you HAVE the money and WANT the extra performance, get the GT...if your upgrading from something rather old but don't need top performance cus your playing Tribes (some may not consider a gt top, but its up there) get the GS.
Im pleased with my GS. The real test will be is it good in a year. My guess is yes it will play everything in a year, but i may need to play with some settings at medium/high instead of max.
One last thing to consider is the frequency of your upgrades. If your the type of person that likes to upgrade every 1 - 1.5 years, i would say go with the GS and wait a year and a half and buy a newer card that trumps all the current cards (but of course probably doesn't beat the the best at that time). If you don't wanna upgrade for a long time, that is another reason to buy a better card now. Also, eVGA has a step-up program where you can buy a GS and upgrade to a better card if you want to in like 90 days (not totally sure of the specs, but its something you might wan to consider).
Anyways, im hardly a computer expert, im just trying to help you rationalize your choice. Either way youll probably be pleased.