Performance wise, theyre both blow for blow the same. The amd might edge out a few fps better on some games with the gtx better by a few fps on other games. As far as mantle goes, its slightly better at some games, nvidia driver makes it slightly better at other games. But that said, the new direct x12 will be the biggest performance boost to all games that will support it, and that direct x12 will be available for both cards. Nvidia has physics and cuda which isnt available to amd, which benefits nvidia only on that route. But cuda is for workstation type programs, while physics is gaming wise. Again, small benefits. Certain programs take full use of physics and cuda, but some programs fully utilize amd's open (cl/gl/cu cant remember).
But overall, amd card is great, runs warmer and uses a tad more power, nvidia is more efficient and runs quieter, neither will really effect performance, theyre equal at that.
So, i personally go nvidia as its quiter, more power efficient, and i need cuda. But cant go wrong either way. Just make sure if you go nvidia route you go 4gb not 2gb. I think the 280 is 3gb. Games, non modded games, just started using full 2gb and some now can use up to 3gb. While right now 2gb is still fine, 4 gb should offer less limiting in the future.