1080 is roughly 10% better in (some) synthetic benchmarks. Compare them here - https://www.nvidia.com/en-us/geforce/products/10series/compare/
Whether that's worth the extra is your call, and based on your own budget and gaming needs.
For what it's worth, the 1070ti is more 'bang for buck' efficient.
Edit: Personally, I don't need that level of performance, but if I was shopping in the 1070 / 1070ti /1080 market, I would choose the 1070ti out of the three.
Upgrading within that group? Mmm. Not worth the money imho.