Both gtx 960 and r9 280 support DX12.
960 is slightly faster than 280 it's actually somewhere between 280 and 280x, but in my country 960 is 60eu more expensive.
So there are 2 options,buy the expensive card and consume less power or buy the cheaper one and consume a little bit more power.
If you can find a good deal on the 960 then go for it.