I;ve looked at a billion benchmarks because i'm in the same boat, but i am looking to step mine up to either one of the cards. When you look at comparisons in performance in these DX10 "patched" games, all our cards have been getting owned, but, lets say for example we have Lost Planet in DX10 at 1280x1024 with everything maxed, except for shadows and AA.
When you are running damanding games like this, extra performance does make quite a difference. Here at 1280x1024, the GTX is getting 12fps more than the GTS, but its only getting 42fps (mind you these are older drivers), so this means that the GTS is dipping down low when it does, and IMO 12fps at these low fps can make a pretty massive difference.
Those extra stream processors are proving to make quite a decent difference and if you are fond of eye candy at 1680x1050 then i;d suggest the GTX. You can do a simple bios flash to an OC version and get ultra speeds and getting almost 20fps more than the stock gts and still only gettign 50fps. Games like crysis will be well optimized but I have a feeling the frame rates we will be getting at high settings will be similar to this and that extra boost will come in quite handy.
you might not agree, but i'm convincing myself of this and getting the GTX in a month or so. I think you could agree that for $137 and getting 18fps+ on demanding games is not such a steep price to pay considering the price you pay extra for 200mhz on CPUs. that's my take on it.