Staying with the i5 is the better choice. With my i7 I only us about 40% of it on load. The i7 is not only for gaming but a wide area of programs that has nothing to do with gaming.
If I had know that at the time of this build I wouldn't have bought it and put the extra cash into my GPU's. So If all you are going to do is game then stay with the i5 and it is a quad core as the i7 is but the i7 has 8 threads.
So save the money and get put it into a GPU or even two of them.