I think Google+ will improve your site rank and Facebook will also increase your site rank not more than Google+.
i think facebook will be the best option for you.because facebook is the most popular social media network in the world and millions of people visit or use facebook everyday.so there will be a lot of possibilities for attracting more visitors throgh this site.
I really like google+, but part of my quality ranking has to do with how many members the site has. Hell, based on customization, I think MYSPACE is pretty good..... but myspace sucks because no one is there anymore. Generally, I like facebook more because of its large number of users and the fact that it seems to be more creative in real usability.
I think both will help you for site ranking but may be Facebook tools will helps you more than the Facebook.
Google plus is google product and offcourse google will give more prefrence against facebook to its own product.
It depends, from the site audience as well. If you want to bring more visits from the google, then Google+ plays vital role in the SEO. On the other hand if you want to engage more on facebook, with interesting posts, sometimes competitions there is a wider audience on Facebook. But a mix of both is always useful.