Atlassian recently added a page where they compare their Bitbucket offering to GitHub. They compare the two offerings based on price and functionality. As well as highlighting their functional strengths, they do acknowledge where GitHub has functionality they don’t.
GitHub offers functionality such as side-by-side diff comments and code search that Bitbucket Cloud does not. We are currently working on bringing support for code search to Bitbucket Cloud. [Updated October 2016]
GitHub offers functionality such as full side-by-side diff, code search and Git Large File Storage (LFS) support that Bitbucket Cloud does not. We are currently working on bringing support for code search and Git LFS to Bitbucket Cloud.
Bitbucket vs GitHub Pricing Comparison
Source: Atlassian Bitbucket – Bitbucket Cloud vs. GitHub
Bitbucket vs GitHub Feature Comparison
[Updated October 2016] – Old Comparison Chart from June 2016