GTX 570 is more powerful in general, but due to the 2gb memory on the 6950 is better for higher resolutions. So in my opinion,they are both high end cards and both would escalate incredible in CF or SLI but I'd go for the 6950, because is cheaper, and I like eyefinity for gaming. At the end it depends what are your main uses such as gaming, Folding etc...
While 2GB might be better for high resolution, even at 2560x1600 the GTX 570 outperforms the 6950....http://www.techpowerup.com/reviews/HIS/Radeon_HD_6950/29.html
Granted its only 5 percent, but it is worth mentioning. And its also worth mentioning I don't think 60 dollars is worth the 5 percent, but I figured it should be clarified.
Now when you are running triple monitor setups its a very different story.
1.2GB is plently of memory at this current time for 1920x1200.