Today we already show the one you use most often. This doesn't seem to be the case - I have 4 mercurial repositories and 0 git repositories, but I'm still shown git, not hg.
I use both git and hg quite often, and fiddling with a dropdown box every time I want to change between them doesn't sound very appealing. Showing the hg link if the user has a mecurial repository sounds reasonable to me.
Alternatively, I think combining both into a generic
reposlist makes more sense. When I'm looking at repos online, I don't really care which VCS a particular repository uses, since I'm not manipulating anything, just viewing.