I initially decided on HG because I'm a Python developer, and a lot of other people in the Python community seem to recommend it (plus, HG is written in Python). Bitbucket had a free plan with a private repo, so I signed up. About the same time, my employer started migrating from Subversion to Git.
For me, the two features I miss most in HG are the staging area and cheap local branching. There plugins that give you rough equivalents, but I haven't used them and it's much nicer to have what you need out of the box.
Thanks for the tips! MQ and bookmarks look like they could be a solid fill-in for Gits staging area and cheap branching. I'll definitely look more into them.
(I've been using git for a little bit but wonder if Mercurial could be more powerful but less popular, and that I'd be missing out)