Google are marketing google+ agressively and have recently implemented their google+ network into organic results. I would use facebook for now but definately invest and establish yourself in google+ as it gains popularity.
I definitely think Google+ is becoming a more viable option and should be used more - it's been showing great results for me personally.
As for the moment, Facebook works great for me. I get a lot of traffic through Facebook Fan Pages and the reach is really awesome.
Google+ all the way. Which is bigger at the moment? Facebook. Which is growing faster? Google+. Think of it this way, if Google has a choice between ranking Facebook or it's own Google+ content first, what's it going to do? Yes look after itself. In January of this year Google reported they were at 90M users, and now in Feb an analyst says they are at 100M with projections of being at 400M within 12 months. I am backing Google+ all the way. It has much better sharing abilities for one (being able to show only certain groups of users specific content). If you are in it for the long term then for me it's Google+ all the way. Cheers
Google plus and Facebook both are good for marketing. For better success you need to use both. Google plus is a wide and vast growing social community which may bring much visitor lead for you. And Facebook obviously the most popular and proven social network site which can bring much benefit for you.