They are both the same. Its just that they are competing with each other for example:
GTX 570 = AMD RADEON 6970
GTX 590 = AMD RADEON 6990
GTX 560 = AMD RADEON 6950
GTX 550 = AMD RADEON 6870
Both the graphics card are similarly priced its just that they are competing with each with similar products. (BTW NVIDA pays most games for there driver support, and most games in 3D needs bullshit NVIDA Cards).
There was a site that compares GPU benchmarks but i cant remember it, just google it if you want it.