If you want to look a spec comparisons, then look at this one
BHP Strix 1080 Ti VS Zotac AMP 2080
Generally the 1080 ti will have less CUDA cores but lower clock speed - you'll find on userbenchmarks that generally the 1080 ti is the slightly better performer, but it can depend on the application.
You'll also typically tend to find that more performance requires less power with newer generations. And overall you'll probably find the actual FPS difference between the 2 to be minimal. But the 2080 probably gives you more "future proofing" / performance in the long run - for want of a better phrase.