While purchasing two cards may seem gnarly, it really wouldn't be better than a single 1080.
First, you won't have to fuss around with getting SLI to work properly. Not that its a massive pain, but it is a pain.
Second, some games do not even support SLI properly. Now this may not happen often, but imagine your shock when your $1000+ GPU setup won't play nice with certain games. (Don't get me wrong, I know most games support SLI, but this doesn't change the fact that there are some compatibility issues with certain games...)
Third, power consumption. It may not be a huge issue, but its another issue to stack on-top of the other issues that come with running two cards. (~150 watts per 970, vs ~250 for a single 1080)
Fourth, resale value. Not that you'll want to sell, but this is still something to think about.
Fifth, as far as numbers go, you're looking at about 20 percent more performance out of the 1080 for less work and you don't have to worry about all of issues that I've mentioned.
...Now say... you want to run two 980 TI's in SLI... I'd have to reconsider my answer entirely at that point!
But if you want one takeaway:
Two 970's running in SLI are not better than one 1080.