Well, if you are using git for all your projects and you are reasonably familiar with it I would suggest to use the git mirror, as it seams to be reasonably up to date. Git should also be faster than using Subversion. If the mirror goes away / gets outdated you can always just svn co / svn up to get the current source.
But as you also have Subversion installed - you could also just use it instead ... doesn't make a big difference and is more a question of personal preference.
Git-svn is a little more tedious to setup and keep in sync - you have to fetch the subversion updates first using git svn fetch and then doing a git svn rebase to apply the changes to the git repository. It will also use up more disc space. This is somewhat more error-prone and I would only recommend using git-svn if you actually wanted to do development on the source tree. If you feel adventurous and want to give it a try have a look at this Tutorial - i found it quite useful to get going with git-svn.