For solely DX12 game, a mid AMD card should be a better choice then Nvidia card. Especially if we considering DX 12 is so new, AMD card most likely can have further benefit when the software is better optimised in the future.
On the Nvidia side, the GPU always perform regardless DX version (no bottleneck from the driver), so, it's hard to believe that the Nvidia card can get lots of performance boost from DX12 in the future. For Nvidia, you pay for it's driver, not the hardware, which is not the key factor in DX12.
So, if you want a GPU that can support DX12 but you still playing a lot of DX11 (or even older) games, Nvidia card still the better choice. But if you don't care about other games, and only want a card has best performance to cost ratio in your DX12 game. AMD is the way to go.