I am currently looking to start a new build. My old build has a GTX 970 so either of these would be a nice step up. I checked Benchmark website that from what I read " In fact, the 1070 Ti is basically the 1070 but with 25% more working CUDA cores (2432 versus 1920) and slightly higher base clock of 1607MHz (versus 1506MHz in the 1070). This translates to the 1070 Ti being as nearly as powerful as the GTX 1080"
Another article I read, which didn't really consider budget/price said if you are upgrading just aim for the GTX 1080 when comparing to the 1070 Ti.
Now the main reason I am considering is I have found a pretty good deal on a GTX 1070 Ti which would nice to save a few bucks towards buildings the rest of my build.
On the other hand GTX 1080 does still outperform the 1070 Ti by a small margin but not sure if the 50-100+ USD jump is worth it?