Depends on the games you play and your Nvidia/in-game graphic settings.
A GTX 1060 (or your current GTX 970) is good enough for basic 1080p gaming on a 60Hz monitor. Some selected AAA games (GPU-hungry games) can't be maxed out (Ultra settings) at constant 60fps.
Consider the GTX 1070 if your looking at those types of AAA games maxing out/at constant 60fps or at ~100Hz+ / ~100fps on non-AAA titles in 1080p.
Consider the GTX 1080 if you'd be gaming on max. settings at 1080p / ~144Hz+ and preferably with a G-Sync capable monitor.
The GTX 1080 Ti, for me, is overkill in 1080p. I'd only consider the Ti if your looking to game at 1440p resolution at 144Hz+++ (or on 4K resolution monitors).