Other factors to consider are if you are using eyefinity amd is usually a better option but you would probably want to be crossfiring to get acceptable framerates either way the extra memory will also help.
Nvidia is usually better in conjunction with 3d monitors and works well with certain adobe applications.
Impossible to say which one is better unless you state your intentions.
I would say neither when $250AR/free ship nabs u an OCed HD 7870 http://www.newegg.com/Product/Product.aspx?Item=N82E16814127662
A stock HD 7870 sits between a GTX 570 and GTX 580, closer to the latter while being mind boggingly more efficient than both @@