I'm pretty sure the GTX 1060 6GB is pretty close to a tossup on mining performance for most coins vs. the GTX 980, the GTX 1070 DEFINITELY blows the 980 away across the board.
The only way I would even THINK about a GTX 980 is if it was very cheap - like $150-$200 OR LESS level cheap - and even then I'd probably take a pass on it.
Gotta agree there. I had a 970 sitting in an HTPC when I started mining. I ran it for a while but did some math and it was better to sell the card for cash on ebay and reinvest that into a 10 series card, the sol/s on the 1070s is almost double a 970 with less power used.