I just got a 760 GTX, for $220 instead of a R9 270X. Why? Because it performs better than the R9 270X when they are both overclocked. G-Sync seems to be a more final product than mantle at the moment, I'll gladly switch to AMD in 4 years if mantle is used in a decent number of games I want. But right now mantle doesn't have any hard numbers and hasn't even been demonstrated on a game.
Also temps, cooling, and noise.
Also temps, cooling, and noise.