I guess it depends on what you are running and your budget.
Nvidia cards tends to produce more FPS with less frame time variance at lower resolutions. While AMD cards like the 512bit 390 and 390x have the extra bandwidth and memory to do significantly better at higher resolutions.
I would lean towards a 980Ti if I were going for a single GPU solution right now and I wanted it to last several years. However, we aren't terribly far from a process shrink and new architecture from both AMD and Nvidia. So depending on what you have now it might be a good time to wait.