It depends on your audience, but for organic marketing, almost definitely Facebook right now. Far more people use it actively. That might change, though. Facebook has consistently decreased the visibility of Page posts on feeds to the point where some people have started to deemphasize a little. (Mark Cuban had a rant on this in November of 2012 though he's changed his tune recently)
For SEO, possibly Google+. But Google can't just ignore activity on Facebook. It provides valuable insight into popular trends. However, Google has far more access into activity on Google+ than Facebook so it's more likely activity there will result in search rank improvement.