They both a completely different technology, it a lot better to go with a Nvidia card due to the fact that you can slight overclock it to beat the R270X.
You can go to any forums ask this same question and without a doubt most will tell you based on how much you can spend and the performance.
If you have the extra money its best to go with the GTX 760 because of its high performance technology. Whereas the R720X will only be really good for games that have mantle since mantle only works with AMD cards.
The choice is still up to you. Weather you have the extra money or not. If you did then it would've been good to get the GTX 760 SuperClocked.