ha exact same boat I am in!
I Kind of have the money for Ti version but I just don't want to buy one for the sake of an extra 20-30fps in games, I am just lumping with this 1080 I have now until the next range of gpu's from Nvidia .... yes that could be many moons away but hey!
Like as Chemma said above no one can make the decision for you apart from give you advice and the advice i have received its got me down from that fence to the ground of 1080's and sticking with it, I don't believe there has been much news on Volta apart from its a possibility of next year but probably late next year for the aftermarkets cards
Chemma, there are people out there like me if they have an option for "utra" for example and the games runs slightly off it bugs me and you want to be able to play it at ultra with no effects at all ... yes i know let me come back to the real world no rig runs perfectly with everything at full whack. Another example my OCD is bad I cannot relax until my whole place has been cleaned other wise if left alone it just bugs the hell out me ...
good luck!
ps you haven't really stated why you want to upgrade just only to assume 1440p monitor with a 1080 just under does it ...whats your specs and reason ? and which games