I really like the comprehensive nature of this article on the HTTP vs. HTTPS debate for SEO now that Google says web sites that make the switch could see a rankings boost.
See: “The Big List of SEO Tips and Tricks for Using HTTPS on Your Website.”
“The degree of the boost is relative. You might not even see an impact – or rankings could drop.
In most cases, I’d still recommend avoiding the HTPPS because of risks with rankings and the fact that any ranking gains could be nominal. Plus, the article notes that your web site configuration could trigger some browsers to post a warning to visitors.