I started using Mercurial because the basic interface and operations appeared more natural to me. Also, at least at the time, the Windows support was better in Mercurial.
After working with some repos using Git, I fell in love with the staging area, and the ability to selectively stage parts of files. I am sure Mercurial has something similar, but it was built into Git.
The only thing that bugs me is that the Windows port appears to be stuck at 1.9.5 (yes, I know it's open-source so I should go fix it instead of complaining).