Git an Mercurial are so similar. You have to remember that Git, Mercurial, and Bazaar were all released within a month of one another to do basically the same thing (because BitKeeper took their licenses away from linux debs). They all have very similar concepts and try to solve the same problem in near the same way. Each of these DVCSs have stolen concepts from one another and there isn't a lot differentiating them. Sure, syntax and terminology differ, but they all end up working pretty similar to one another.
You have tools like Kiln Harmony[0] that lets you use git/mercurial for the same repo (looks like it cross-commit between the 2).
Git gives you all commands all at once, while Mercurial gives you just the functionality needed 90% of the time that can't really get you in trouble. Mercurial ships officially supported extensions with Mercurial that let you do history rewriting and other fun stuff that people are used to with git rebase.
I'm more a fan of Mercurial's CLI syntax/terminology, but Git has really won the popularity contest at this point. If you ever have questions about Git usage, the question has probably already been asked and has a lot more people that know and enjoy it.
Mercurial does have some advantages going for it still I believe. Being purely written in python makes it work a bit better on Windows. And as Facebook has said, it is easier to develop new plugins for Mercurial (at least given their skillset). I also like the Mercurial Largefile extension (if you have to deal with a lot of binary data). I know there are 3rd party modules like git-annex, but nothing officially part of Git.
[0] http://blog.fogcreek.com/kiln-harmony-internals-the-basics/