For webwork system code (i.e., the webwork2 and pg code), the git version is the most up to date, and it will stay that way. The svn repositories for webwork2 and pg are frozen. I know that is not what you are asking about, but I wanted it to be clear that the difference between those parts of webwork and the OPL/NPL.
For the time being, I recommend using the svn version of the NPL/OPL. It is up to date as far as bug fixes, and is the one being actively maintained. As far as I know, the git version is at least 6 months behind. I don't know of any bug fixes which are in the git repository which are not in the svn respository. So the choice seems clear.
Switching back to the svn version of the NPL should be easy (set a symbolic link, set the library version number in a config file, and run OPL-update), and I think Mike has already covered the steps in more detail.
At some point in the future, the NPL/OPL will switch to git and freeze the svn repository, but I don't know when that will be. It would make sense for there to be an announcement in this forum when it happens.