Redheadsrule13 :
If you're just going to be playing BF4, then I'd recommend the R9 290X, it's under $300 and it should perform nicely with Mantle. However if you want to get something a little more expensive, but also more powerful, go with the GTX 760, it'll perform better than the 270X in non-mantle games, and is still under $300.
It depends on how long you plan to use whichever card you pick, and what other games you might want to play with it.
I'm guessing you meant the R9 270X, right? xD
And I agree with the above posts. If the only game you're really interested in is BF4, then the 270X would work great. If you're interested in a better overall card though, go with the gtx 760.
My personal preference would be to go with the gtx 760 though - I personally don't feel like mantle will be worth the difference in overall performance between the two cards. The 760 is also the best bang for your buck card in the 200-300 range.