Depends on what you're going for. The R9 is a more powerful card, but the 960 is more efficient. Potential driver issues aside, the R9 would likely deliver a better gaming experience in most cases, where the exceptions would probably lie in anti-aliasing and higher resolution output, where the 960 would probably do marginally better.
Overall, I'd say go for the R9 280, since the efficiency of the GTX 960, in my opinion, doesn't provide more benefit than the performance gains you'd get otherwise. I'd rather have performance at my disposal and not need it, than need it and not have it.