Benches look neck and neck on those two. I'd go with whichever is cheaper or comes the best gaming bundle you'd actually use.
I would narrow it down to these two. The 7870 XT wins in price and I think it will be a little more future friendly. Otherwise they are very similar. I play WoW, Borderlands 2, Diablo 3 and Civ V all at max (WoW is tweaked a little) at 1920x1080 while carrying over 50 FPS easily on the AMD card in my sig. so I wouldn't let what card one game or another does a little better on sway you too much.
SAPPHIRE Radeon HD 7870 XT w/Boost 2GB: $259.99 + $15 rebate on Newegg; 256 bit; 975mhz core
MSI N660Ti PE 2GD5 GeForce GTX 660 Ti 2GB: $279.99 + $20 rebate on Newegg ; 192 bit; 980 mhz core
PowerColor HD7870 PCS+ Myst Edition and it comes with 2 free games...you can sell em and with the money you save plus the games sold you could get a SSD.