I'd say go for the 280x or an 80s series on AMD's side. benchmarks using DX12 seem to favor AMD cards. Nvidea cards tend to suffer a performance hit when they move onto the next architecture, the old kepler cards suffered heavily when Nvidea moved to Maxwell.
update: I looked into it and it looks like Nvidia released a patch to fix the issues on kepler, though I would still go for the 280x unless your power supplly can't handle it.