both cards are so similar it's hard to choose, just pick whichever card you like best, both will run great on that PSU wattage. Just comes down if you like Nvidia or AMD, but knowing me, I'd choose The AMD R7 260X.
Main advantage now going for AMD graphics cards is Mantle, which is quite a good reason to go for it.
Also, the R7 260X is able to be crossfired (2 cards basically running as one), whilst the GTX 750Ti can't SLI (Nvidia's name for using two cards), that's a pretty good reason to also consider it.