Github → Competitors
A development platform for everyone.
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).
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.