Couple things...
Here are the basic rules I try to ponder when thinking about upgrading a gpu...
1. Buy the single best card that is in your budget. Maybe even a little over your budget
2. Never use future sli to talk yourself into a card. Thats my rule after using sli and I won't ever go back. For me, if I cannot afford a single card solution that will accomplish what I want, I will wait and save more or wait for better tech
3. What do I want this card for? 120Hz 1080p? 60Hz 1440? 100Hz 1440? 60Hz 4k? Am I ok with turning down some settings to hit my fps targets?
4. Is my card limiting me right NOW? Am I getting frustrated with performance or low settings?
Here is the deal with the gtx1080. People have problems with it compared to the 1070 cause the price to performance. Its an expensive upgrade from the 1070 without huge performance upgrade. The general buzz is that its sorta low value. Thats why when you bring up the 1080, people are prob always gon start talking about the TI. I am not sure if the value is really there with the Ti but its a pretty big jump in performance. A bigger jump than between any of the other tiers.
But then if $700 is just plain too much for what is basically a toy, that is totally fucking reasonable and if thats past your limit then everyone else can fuck off with their opinions.
As a gaming enthusiast, personally I cant fucking stand fiddling with settings and the ini and whatever other bullshit to find that perfect balance between fps, frametimes, and graphics. Its worth it to me to spend more and avoid most of that shit. So I will pretty much always go the best I can afford. I would spend the extra to go 1080 over 1070. But that doesn't necessarily make sense for everyone.
Lemme say this about the Ti though... I am still running the 980ti that I got almost 2 years ago (lowly 1080p but still) and I have never had to turn a setting down. I bet I will be able to totally skip the 10XX generation and when the 11XX Ti comes out in another 18 months or whatever, I bet I will still be close to maxed settings on 1080 and 1440 @ 60 fps both.
Finally, if you are thinking at all about sli, fuck that and just get the Ti. But you know, just digressing now.