If he plans on running ultra settings with AA then as of right now neither of these cards is good enough.
The cards generally perform about the same with the slight edge going to the hd6950. With the AMD card, you get the extra ram which may help some of the time. With the Nvidia card, you get Batman Arkham City for free. It's my personal opinion that Nvidia has consistently better driver support and developer relations.
EDIT: If you don't mind mail in rebates,
http://www.newegg.com/Product/Produc...82E16814162075 then this card ends up being the same price as the hd6950, outperforms it, and comes with Batman AC.