if you plan to go the 4GB, it will only make a difference if you go Crossfire down the road, if not then the 2Gb vs 4GB will be exactly the same. If you want proof to this, I can (Even for 4k, multiple screens and when games require more then 2GB Vram).
The GTX 760 is still the better choice.
mantle is only a game changer in games that have their engine, and from the top of my head that's only two games. Physx engines are in far more games and more modern day games, making Nvidia have the better drivers.