they are almost identical in terms of performance. they trade blows at the top. overall, i would go with the 6950. but the best thing you can do is see which performs best in the games you intend to play.
Omg... Are you planning to get one NOW? No no no. Have a little patient, dude. Gtx670/670ti may roll out in a few weeks. Old card's price will go down and you'll have more choice to choose from.
Btw I have the Evga 560ti FPB 1GB OCed it to 1000/2000/2200 @1.075v. Pretty awesome but I'd take the 6950 2GB over the 560ti every time if money is no object.
The Sapphire 6950 is a very good card. I like Saphire for their heatsink designs and low noise fan. I was looking for that card about 2 1/2 months ago (before I got the 560ti) but it was sold out. It was $270 back then but now it's on sale for $230 on Newegg last couple days. If your price range between $200-250, that's the card to get ONLY IF they didn't lock the bios down on this batch.
they are almost identical in terms of performance. they trade blows at the top. overall, i would go with the 6950. but the best thing you can do is see which performs best in the games you intend to play.