Two 970s would certainly beat a 980; not sure about a 980 Ti though, and do remember the VRAM capacity difference.
I should imagine that unless you push the detail settings, resolution, modding options, etc. to the point where much more than 4GB VRAM is being used, you'd be very happy with two 970s. 8) (I would!) I had an older setup for some time which was in some ways kinda equivalent with respect to how performance demands evolve over time, namely two 3GB GTX 580s. I was playing games that didn't exceed the VRAM capacity (Crysis), but upgrading to a single 980 did provide the extra performance I wanted, while for newer games (Elite Dangerous at silly high detail levels) the higher VRAM definitely did help.
Thus, all of those...