GTX 760 Ti is a rebadge GTX 670 and R9-270x is a rebadge HD7870 for the retail market, AMD calls that same card HD8870 for the OEM market.
Out of the two cards GTx 760Ti or R9-270x, get the GTX 760Ti
Is there a big difference in performance? I also plan on getting the x51 and I cannot decide which card to get. I would prefer to use eyefinity over Nvidia surround, so is there a slight or big difference between the two?