There is little technical reason to choose Git over Mercurial or vice versa. Their usage is extremely similar. Git exposes a few more internals, and lets you fix mistakes more easily. For sharing branches which are incomplete, git's `rebase -i` is also somewhat superior to mercurial queues.
Overall, I've found git to be marginally better than hg.
The real killer feature that git has is that git talks to github and svn. Mercurial only talks to bitbucket (sorry guys, network effects are important and github won).
The genius thing about git-svn is not that it only "talks" to svn: using git-svn (you could say subversively) converts your repo to git. One day, when somebody suggests dumping svn for git, you can skip the "but how do we preserve all that history if we switch" conversation, because it's already done.
Mercurial also talks with git (http://hg-git.github.com/) so you can use it with github if you want. This was actually created by Scott Chacon, the guy who wrote the subject of the OP.
I think it's worth giving both git and hg a try to see which fits your head better. Mercurial feels better to me and there are quite a few people in many communities (including most of the python community).
Overall, I've found git to be marginally better than hg.
The real killer feature that git has is that git talks to github and svn. Mercurial only talks to bitbucket (sorry guys, network effects are important and github won).