The GTX 760 exactly. the 270 (non X edition) is a bit behind HD 7950. 270 is the same as HD 7870. The GTX 760 is a bit better than HD 7950. So the concusion is get the best performance for your budget, and the GTX 760 is the winner. For mantle it will give more preformance but not quite that much. Still if you get GTX 760 it will be better than 270.
Hope this will help.