SSL is also a trust factor thing and has been standard and customary for several years now.
Not having an SSL may have not only have a negative impact on your rankings but it will leave a negative impression with site visitors who look for that locked padlock icon to know they are on a site that understands technology -- and that they can trust.
So, SSL really isn't a choice -- it is a necessary part of having a website.
I'd always advise using an ssl, even a basic version. This is NOT essential and your site WILL still rank, however visitors will receive an insecure warning which can be quite off-putting to some viewers. A secure site and secure content will increase your google rank but there are many factors at play. Don't think an ssl will send you to the top of google without a multitude of other methods in place.