Nothing wrong with SLI. I've been running dual 1080s for the last 3-4 years, no problems at all. If you can add a second 970 cheaply to tide you over till Ampere hits, I'd say go for it.
I had SLI years go and it had me tearing me hair out, but I couldn't go to 1 card as I had spent a fortune on it.. All I can say is I wont be going back there ever EVER again. When I eventually Switched to 1 card, it was like heaven as all the annoying problems just disappeared and the pc just worked.
I like my pc's just to work with no faffing around