Well, neither, until you tell us your specs. I will need to know your system configuration to know what to recommended. For the best price/performance ratio, get a hd6950 and unlock into a hd6970. Again, only if your system can handle it.
Unless you are against AMD, grab a hd6950 and unlock and OC. The GTX 570 is not worth the money, and the GTX 560 will be beaten by a unlocked hd6950 in most games. To unlock a hd6950, you will need a reference version with 2gb RAM on the card. Like this: http://www.newegg.com/Product/Product.aspx?Item=N82E168... If you must go with Nvidia, get a GTX 560... Hope this helped!