To be honest, all of of these cards are a bit of overkill for current games at 1080p and you should be more than satisfied with either the R9 or GTX 980 (non-Ti).
That said, nVidia's current DX11 (and below) driver stack does give them a relative performance boost at lower resolutions.
Relative in that you will still have high performance out of both cards, but the R9 is, on average, matched by the GTX 980.