Depending on your resolution the 6950 2
GB 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.
Id get this.http://www.newegg.com/Product/Product.aspx?Item=N82E16814127594