Best performance = GTX 760
Bang for buck = R9 270X
Alternatively = R9 280 (Non-X)
Generally the GTX 760 beats the R9 270X by a few FPS, and the R9 280 beats the GTX 760 by a few FPS.
The biggest advantage iv'e seen the 760 have, over either of them, is in the Batman Arkham Origin game, by some 20 FPS.