GTX590 or R9 280x for Blender

Hello, first time poster here!

I recently got into Blender and did a few tutorials, I love it so far! I read dozens of threads about how your GPU can speed up your render times significantly. I was really looking forward to blazing fast rendering times until I learned that my HD5970 is not supported in any way. I plan on replacing it for a card with great performance in Blender for a good price. I can get a GTX590 for cheap and a R9 280x for a little more. Since Cycles performed better with CUDA on nVIDIA than OpenCL on AMD, I’d guess the GTX590 is the better choice. However, the R9 cards look very promising because of the huge improvement in optimisation only recently. My question is: what is the better option? which one performs better? A slightly cheaper GTX590 or the 280x? Is the difference in VRAM really something to worry about?

Necessary specs of my current rig:

AMD Phenom x4 965
AMD HD5970
4GB RAM (yes I know, next thing to upgrade)