I played a bit of BF4 again and I find it a lot easier to run and gun without consequences. In BF1, I really need to make sure that my angles are good and that I'm not going to get killed by explosions next to walls.
For the DX12 thing, it will definitely be in BF1. As stated before from someone else, will it be at launch or not. I'm hedging my bet that it will be in but probably a bit buggy.
As for it deciding your card purchase, BF's history is that it is optimized well for either vendor. The real question is what other games and features you are looking to give you some value add.