GTX 680 and Radeon 7970 are both great deals. Nvidia just came up with new drivers days ago which improved performance in Far Cry 3 on the GTX up to 38%! That is a lot, so if playing Far Cry 3 and Crysis 3, then I'd go for Nvidia.
Still Radeon 7970 isn't a bad choice. But you also should note, that yesterday Nvidia brought up a new gaming software, that will automatically fit the Graphics in game, so you can get the best performance in every game, it's just a beta though, but it will be a great Tool in the future.
Another Thing is all the Technologies you get with Nvidia FXAA, TXAA, physx, boost and my favorite adaptive vsync.