1080 ti is great long term option, but obviously depends on your budget. There are some great deals on used cards so I would suggest to look for used ones, but be careful of faulty cards. Although it probably isn't a major problem.
There doesn't seem to be any major cards coming out from Nvidia until next year. However AMD is releasing Vega cards so I would keep my eyes open and follow the GPU space.
Yes 1080 ti is great long term option - but when Vega shows on market - price maybe drop
Things must be rough where you live if you consider $540 for a 1080 and $460 for a 1070 a great deal. You could buy two of the RX 470 4GB mining cards for $500 or the gaming RX 570 4GB cards for less than that, if you can find them and get roughly 20-30% more hashrate for the same price.
RX 470 8 gb price is 220 eur and hashrate is up from GTX 1080
It's a bit of a struggle to find things to bet on when the main European club football season has finished in the summer momths so I will follow this pick here, thank you.
I'm very glad I'm useful to you and you like my predictions. Regards