Well both are great and performe about the same so all ai really have to say is get the one that is cheaper and has a non reference cooler + if you can find an hd 7870 xt for the same price as these cards go for that one its better and last time is saw it the same price.
Jaslion: wow dude really? They don't perform about the same the 760 performs right under the 670 and the 270x is just an overclocked 7870. Battlefield 4 and other AMD optimised games they perform close but in GPU neutral games the 760 is well above it. On average the 760 is about 15% better at stock clocks
As for the model the Asus was just on sale and may still be at Newegg for about 140. And try to not get reference cards. The nvidia reference aren't horrible like some of the r9 series reference cards are but just going to the evga acx is a huge step up in cooling and noise
Also, can someone explain if Mantle will make the r9 270x worth getting?
Sorry for all the questions, This is my first time i will be building a computer, so I just want to make sure I get the correct parts.
Noone can tell You that at the moment Lot's of noise but still no life tests of it.. So just get 760 and You will have descent performance for the money not some "maybe better" performance in some limited titles in future.