The main concern with SLI is that it has issues like micro stuttering and scaling is not very good. But now some people say SLI has been greatly improved and those issues have been greatly corrected.
I don't personally know how it works now as I don't have a personal experience with SLI.
Considering you get good scaling with 2×1070s, you must get performance on par with a 1080 Ti.
If you consider buying Volta, I'd say that a single x70 will either outperform or trade blows with a 1080 Ti. Historically, x70 have performed within a delta of 10% (+/-) with the previous generation x80 Ti. (970 and 780 Ti or 1070 and 980 Ti)
So the choice is just a matter of budget and time.
If you want spend less and want better performance right now, get a 1070 and put it in SLI.
If you want to spend less and can wait, then wait for the Volta x70.
If you can spend a bit more, get a 1080 Ti.