So you guys want to turn this thread into flame war? 😀
@OP just pick one that you're comfortable with. With hardly any DX12 title out there it is hard to say which one will have the advantage on DX12. What is your gaming resolution? 390 comes with excessive VRAM but that much VRAM is quite useless at 1080p. Even on high res probably that much VRAM did not guarantee performance advantage unless you're going to use it in CF pair.
If you're going to play at 1080p the memory segmentation of 970 should be not much of an issue.