Performance:
The Gtx 760 performs about the same in most games @ 1080p. The R9 270X will do better in games with mantle support that is supposed to come in new games. For BF4 the 270X will do better
Bottom line: Both cards will suit equally well
Features:
The r9 270x does have mantle. This could be useful in upcoming games.
The r9 270x has a much much much better cooler on it for overclocking
I personally think mantle now isn't useful, a friend who have a r9 270 has mantle in bf 4(the single representative game using mantle) and when he activates it he gets around 10 frames but the screen starts freezing and blurring every 5 seconds and the card performs and works well.
So when mantle will be optimizated and start being used in some games, DirectX12 will be here giving the same performance as Mantle gives.
END ALL:
Get the 270X if you will overclock
Get the 760 if you are going to play at 1080p without overclocking