again heres a simple answer: it depends on your budget
anyone who says get neither and get a 9 series card down the road is a total buffoon imho. When a 9 series card comes out ( assuming it does come out nov/dec ) it will be at a ridiculous premium pricing since it is not only new but the most powerful on the market. If you have the money get a GTX I say since it will last you very well 3-4 years ( the GTX will have a price drop in nov/dec cause of the 9 series release which is an ideal time to buy ) or if your budget get a GTS ( non overclocked version since u can just overclock it yourself ).
Keep in mind that the gap in performance between the GTS and GTX is one of the largest gaps ever for an nvidia series of cards ( usually the gap isnt that much diff like with a ati x800 xl and xt ). It really can mean the differance of 25 fps in SupCom or 42 fps in SupCom ( I prefer the 42, which is why I got a GTX )