760 is about 15-18% better than the 7870 on average. In the most AMD optimized game I know of (battlefield 4) the 760 is a couple frames better but its close enough to call a tie.
So many people keep saying the 270x is equal to the 760 but that's only in a couple games that perform WAY better on AMD. In truth the 760 is 12-15% better. Its just one of those things that bothers me like people saying the 270x is a rebranded 7950 but it's not. its a rebranded and overclocked 7870.
Sorry for the rant