Mercurial → Competitors
A free, distributed and secure version control system.
When to use Mercurial
Irrespective of the project size, Mercurial's decentralized development model can be used by users who are less experienced than pro-developers. This is because unlike other CVS, Mercurial doesn't allow its users to change history.
When to use Github
If you can afford the monthly plan for private repos, Github should be your default choice (given the huge community & large number of open-source projects).