Comparing the AMD versus nvidia cards it will generally change depending on how you use it. R9 series cards are very aggressively clocked in the box, the nVidia ones have much larger overclocking headroom.
But here the 770 is 41% faster than the 270
If you wanna go up tot he 280x, then the 770 still has the edge(101% to 98%)
Overclocked tho, it's not close even in games like BF3 that favor AMD.
Reference 280x = 86.5 fps in BF3
Reference 280x = 91.6 fps in BF3 (5.9% advantage)
MSI 280X overclocked = 95.6
MSI 770 overclocked = 101.7 (6.4% advantage)
Though, I'd wait till September's anticipated release of the 880 ..... that's going to completely rest the price structure.