The 760 is the better, more future proof of the two. But if you are going to spend that much, get the GTX 670. It is a better GPU as it has more CUDA cores, SMX and testure mapping units. There are a bunch of 670s on sale right now. You can get a 2GB 670 for less than $300 which puts it in the same price class as the 760.
but gtx 580 preforms better in 3d rendering programs. will it change in the future updates of render programs like OCTAN?
Yes, that is true. I was assuming you wanted to be able to play better games in the future. But, for 3D Rendering specifically Octane, the 580 is the 2nd fastest only behind the Titan. This is because the newer cards are optimized for games.