A plus1 from Google+ is now one of the strongest weighting factors for SERPs. A +1 has a weighting of .4 where as a backlink has a weighting of .32 (if memory serves) which I think is kinda mad.. A lot easier to get +1s than it is to get backlinks - it's like Googles coup d'etat of the social media world..
I disagree with Rob. Eventhou Google has release many updates. In practice backlinks has biggest seo weight. But releasing update on 22 August 2013 called Google's In-depth Articles it is true that Google+ has bigger influence than before.
Matt Cutts says that G+ doesn't affect Google ranking, however i think it certainly does. All of my post that have 10+ Google plus shares rank very high in Google, while i might have some other post with 30+ likes on Facebook and they aren't ranking as high while generating much more traffic. As of now, i say: yes, Google plus does help to rank better then FB
I tend to get more +1's on G+ then I do on Facebook so I think that using gmail and G+ to promote is better then using facebook in my opinion.
Google Plus is simply setup to let you define exactly what you want to see and from whom, whilst also highlighting some of the best people, content and thinking so you can expand your interests and horizons. Facebookfocuses on connecting you with your existing friends and your relationships with them Google Plushelps you build new connections, find interesting people and discover content that can surprise and delight you And this is just the start; Google Plus is only going to grow and become more popular, it’s time to make the switch, or at least come along and see what’s happening. Two of the most exciting features on Google Plus are personalised search and hangouts. 1. Personalised Search– As we all know, Google is always working to try and provide the most relevant search results for our questions. One of the best measures for this is what our friends and connection think; after all, if it’s relevant to them, it may well be relevant to us. Google Plus uses these connections to suggest search results based on what your contacts on Google Plus like. If they have read and liked an article, it may show up higher in your searches on a similar topic when you do a standard search on Google. This means that you get to the information you need more quickly. 2. Hangouts– Imagine if you could hold a real-time, virtual meeting with video and audio with a chosen collection of friends, colleagues or others. Google Plus has this functionality built in, through ‘Hangouts’. These are virtual meeting rooms that you just need a microphone and (optionally) a web cam to join and are a great way to discuss common interests.
For our system we use both Google+ and Facebook and I can't say that feedback is huge. So at the end it depends on the audience, if there are more people using FB - you'll receive more from it, if there is more people using G+ - it gives more advantage. But on average they are not much difference between them.
I think both are useful for SEO. But in answer to the question which is best then i would say Google+.
For me...barely any of my friends have Google+ and it hasn't seemed to have caught on yet so I'm sticking with Facebook. I'm sure it is good but at the moment there isn't enough interest from people around me for me to push Google+.