Earlier quoted context omitted.
Are there that many non-enterprise companies using Bitbucket over Github? Most startups and mid-sized companies I know use Github for private repo hosting.
I think the integration with JIRA (which is arguably best-in-breed for moderately heavy-duty issue tracking) is quite a compelling argument for going with an all-Atlassian setup; you can even use their SourceTree product as a git GUI. On the other hand, GitHub's stuff generally does feel nicer to use and better and more thoughtfully UX'd.
I assume it has some features that are paid (or only work with Bitbucket?), but I've found it brilliant for managing GH repos - one of the nicest free tools I use.