If you plan to do 4k gaming on one GPU, and want to play all games at decent frames and settings, I'd recommend waiting for the 1080 Ti, and even Vega, which is rumored a couple months later (by end of May).
Keep in mind too that even if you aren't interested in Vega, it may very well keep the 1080 Ti at reasonable pricing. The last thing you want is to make a hasty decision, then end up face palming a couple months later.
Also keep in mind that you want to think about how what you buy will hold up 2 or 3 years down the road.