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To be honest your picking at straws here, its not like there's a massive gap in performance between any of these cards (I include the 6970 in that statement as well). Once a 6950 becomes redundant for games at max you can bet a GTX570 will be as well.
If I were you I would be looking at other features such as included games, warranty and the effectiveness of the cooler (such how quiet it is under load).