Bitbucket vs GitHub

Both Bitbucket and GitHub are a web-based hosting service for projects using the Git version control system.

GitHub provides free accounts for open source projects, for other types of users it offers free and paid accounts. More details on GitHub can be found on its site
https://github.com/

Both Bitbucket and GitHub provide private repositories, which come with a license fee.
More details on the plans and license fee can be found here.

https://github.com/plans
https://Bitbucket.org/plans

Bitbucket supports both Git & Mercurial version control systems, while GitHub
Bitbucket was developed and managed by Altassian.   Altassian provides a GUI based editor for Bitbucket called SourceTree.

GitHub also supports GUI based editors like GitHub Windows, SmartGit etc

GitHub was launched before Bitbucket and a has a bigger community.