I've ever seen a convincing argument to choose hg over git. Sure you can do a lot of things with it and I'm sure it's perfectly fine to use, but if you know git, and you like git, there's no reason to switch. What does hg have that makes it better than git that you should give up git? Popularity is what git has over hg. Someone using git will probably never have to spend anytime worrying about learning hg, but an hg user will have to become familiar with git because git is used everywhere nowadays. So my recommendation to people new to versioning systems is to pick git. I can't for the life of me understand why someone would pick hg for an open source project unless they wanted to throw learning curves and barriers at developers, or they just really really liked hg (probably because they chose it over git a long time ago).
[1] http://mercurial.selenic.com/wiki/RevisionNumber
[2] http://stackoverflow.com/questions/8995598/can-i-mark-a-bran...