My answer 6 months to a 1 year ago would have been SLI but....
Now, I'd suggest you get one GPU. There are too many 2015 titles that released after Q2 with malfunctioning SLI profiles or no profiles at all. What's your upgrade budget?
Also, what is your build specs? Please include your power supply's (1) brand / (2) model / (3) wattage.
What games are you playing or plan on playing? What resolution are you playing in and how many FPS are you aiming to achieve? Generally I don't like the GTX 980 for the money, unless you have 21:9, 2560 x 1080 resolution.
Addendum: Without knowing these answers, I have these thoughts: Upgrading from a GTX 770 to a 970 would roughly grant you 20% to 30% performance improvement, which I'm not entirely certain you would notice. And I haven't personally liked the GTX 980 since the introduction of the 980 Ti. Therefore, I probably wouldn't bother upgrading unless I was willing to at least get a GTX 980 Ti. I would purchase one of those $600 versions of the card and then I would sell the GTX 770 on Ebay to recoup some of the cost.