What resolution will you be using? If it's 1080p I'd just get a single 970 and have the option of upgrading to SLI 970s later, if it's above 1080p I'd go for SLI 970s.
The 970s will be quite a lot faster than a single 980, and for not that much more. I'd say it's worth dealing with the SLI issues in that case.
(By resolution we mean your monitor resolution, not game settings. Also, the 3.5GB issue has kinda been blown out of proportion, so don't worry about that)