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.