Personally I can see the 1080ti being 20% faster than the 1080 and 20% or 30% more money (the price difference goes up exponentially as you get near the fastest card).
Unless you really need that extra 20% its probably worth either waiting until the pascal refresh later this year or until Volta next year. Volta according to the Nvidia roadmap graphs should offer 50% performance boost over Pascal and will be a major leap. Only time will tell if Nvidia can deliver.
I certainly wouldnt be taking a hit on selling a 1080 to swap to a 1080ti.
Unless you really need that extra 20% its probably worth either waiting until the pascal refresh later this year or until Volta next year. Volta according to the Nvidia roadmap graphs should offer 50% performance boost over Pascal and will be a major leap. Only time will tell if Nvidia can deliver.
I certainly wouldnt be taking a hit on selling a 1080 to swap to a 1080ti.