They're about the same: http://gpuboss.com/gpus/Radeon-R9-380-vs-GeForce-GTX-960, I would go for the 380 because it's newer, but it's a good idea to get whichever's cheaper, both can run any game at ultra textures, and high anti-alias...etc.
anti aliasing demands lots of graphic processing and nivida made its card to be better for the use of FXAA,MSAA,TXAA...etc(anti alaising tech)
so if you care much about anti aliasing tech then go for the GTX 960 and if you want a higher FPs without the anti aliasing then go for the r9 380
(this is not enterly right )