First, I would recommend upgrading to 2.17 instead of 2.16. 2.17 was released last July.
The difference between the tags with the + and those without is that the + tags have all hotfixes applied, and the tags without the + are the initial releases. So WeBWorK-2.16 is the initial release of webwork2 version 2.16, and WeBWorK-2.16+ has all hot fixes applied (and was the last commit before WeBWorK-2.17 was released). The WeBWorK-2.17 tag was the initial release of webwork2 version 2.17 and the main branch includes additional hot fixes.
If you don't see the WeBWorK-2.17 tag, then run "git fetch --all". That will not update or change your code, but will update the git refs to the current status of the repository on GitHub.