Performance-wise, DX12 is a lot smoother for me. DX11 is a stutter fest as it’s so CPU-bound in comparison (you can see this in the debug FPS panel).
Appearance, DX11 colours look more washed out and faded blue. DX12 pops and looks a lot better. However the god forsaken ground texture bug is still ever present only in 12. When will they ever fix this?!
Stability, both are fine for me now, but I’d say if anything DX12 I have had more of a rocky road with trying to get to this point (i.e. seems more sensitive to GPU drivers, memory timings/config).