I'd love to see a more detailed description of this, including why someone new to Mercurial would choose one over the other.
I think someone new to Mercurial shouldn't choose either one.
Just sit back and wait for the real migration to happen.
I would say that using the SVN mirror is a fine way to experiment with using hg against the Python sources to develop and test patches.
I think your description already falls into the "advanced user" category. The new-to-mercurial committer should (IMO) use a "what if it still was svn" workflow, which uses hg pull/up/commit/push. That can only work if pushing really has the desired effect, which isn't the case for any of the installations that we operate. When the test-for-two-weeks installation becomes available, the new-to-hg users will have a chance to learn how to use it without fear of breaking anything. Until then, they should just remain patient. Regards, Martin