If the titles you play support SLI and scale well, the 970SLI option mght be somewhat viable.
BUT, SLI support is poor in general, and scaling is never 1:1. A single, stronger GPU would be preferred.
I would full expect a 1070, 1070TI or 1080 to offer similar (if not greater) FPS vs SLI 970s outside of a titles with great SLI scaling.