A nicely factory overclocked 970 will come within 10% of a stock 980 in performance. And if you get a good card with an excellent cooling solution from the likes of EVGA (ACX series), MSI (Gaming series), ASUS (STRIX series), Gigabyte (G1 series), or Zotac (AMP! series), you can overclock it even higher to equal a 980. Like others, the 980 is not worth the 60% higher cost over a 970 for just 10% better performance out of the box.
If you have to get another card now, get one of those, whichever is the best priced in your region. Otherwise like another person here said, I would urge you to wait for the 970/980 replacements (Pascal) to come late this summer. Based on recent generation release history from Nvidia, the 1080 (or whatever it will be called) will be released first followed up by the 1070 a month or so later. And also based upon history, the 1070 will likely be equal to if not faster (with more VRAM) than the 980.
Also like another said here, there has been an upward trend of multi-GPU support not being offered or having poor scaling. For this reason alone I will seriously be looking at getting a single high end solution to replace my SLI 970s for my next GPU upgrade (whatever the 980Ti replacement will be).