There was a nice post about Atlassian's choice of Mercurial over Git on their blog: https://blogs.atlassian.com/2012/02/mercurial-vs-git-why-mer... Their argument about a windows version doesn't seem to hold today. There seems to be a first-class Windows version of git.
Since there exists HgGit (http://mercurial.selenic.com/wiki/HgGit) and git-remote-hg (https://github.com/rfk/git-remote-hg), the two are equivalent in the sense that you can use either to contribute to any hg/git repo. Which is your prefered flavour of interface?