Oddly enough in recent articles from Tom's Hardware they are saying that essentially the two cards have very similar performance. Then it just comes down to preference, prices and options.
So, do you prefer AMD and their Mantel tech, real hair tressFX or would you rather nVidia's PhysX and etc.
Other things you might consider is how much power does each card use? Does either card come with a free game?
I realize that is no help at all...