This is just me guessing and postulating here, there could be a significant difference between them just in a way that I haven't anticipated.
At the $250 price point, your options are a 760, a 7870 or if you get lucky a cheap 7950.
So in realistic terms, your looking between a 7870 and 760.
Right now (because who knows if AMD pulls another 12.11 out out nowhere) the 760 is the superior buy, unless AMD's Never Settle bundle is enough of a lure for you to overlook the ~18% gap in performance between the two.
http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_760/27.html
Even looking at the odd-ball option of a Tahiti based 7870 (basically the GPU of a 7950 with the memory setup of a 7870), your still going to be trailing the 760 by ~10% at my guess.