There was debate in an unrelated Skype chat tonight, where somebody ultimately tried to conclude that the upgrade wouldn't be worth it, and that the AMD Fury X performs better (there's only 100 SP GLOPS difference which is minuscule honestly).
I'm still getting the 1080, replacing my 980. It's 20% stronger than the 980Ti and well above all the previous Titan models. Besides, AMD doesn't get access to PhysX or any of the GameWorks features, and I've just had some bad experiences with AMD cards in the past.