Depending on your resolution the 6950 2GB is about 2 percent more powerful then the 560 Ti. The 560 Ti has Nvidia Physx backing it though. I'd get Nvidia, becuase I like Phsyx. However not many games use that so it's not a big deal.
It really comes down to price, they are on par with each other.
If you prefer Nvidia go with the 560Ti. They are pretty much the same the 560Ti and the 6950 power wise and trade blows with each other. Its all up to preference.