only reason for you to go with an AMD branded card at the moment is for MANTLE... but Mantle itself seems to be crashing down at this stage. I would personally go for the Nvidia Card!
If you do have a lower-end PC(i3 or under), and you're only interested in playing BF4 or Thief, Mantle might be able to help you out a bit though. As the Forums suggest though, it's not to be long lived.
AMD's R7 260X also offers support for crossfire, this might come in handy if you do want to add another GPU later on(instead of buying a card that costs 2x as much.)
Nvidia has the upcomming G-Sync on it's side whih seems to be very promising, but that's going to cost you a new monitor too.
I'm not being bias, but Nvidia seems the way to go..
I had a 650Ti last year and replaced with with an HD7950(managed to get it for $250)