What the 2 previous members should've asked first is what is your full system spec??? (especially which CPU are you using). Because without this information you could end spending a lot of money for nothing if your CPU can't handle a 1080/1080ti.
With that being said, and IF your CPU/PSU/memory are "strong" enough for it, if I was you, I would wait for the release of the 2070 to see how good they will actuallyperform... If you're lucky enough, you could even see a price drop for a 1080/1080ti... ;-)