Since BitBucket is run by Atlassian, whose primary business (as far as I know) is enterprise software, I would guess (again without knowing) that they're in a pretty good position.

They're well known for their wiki, Confluence, and their ticket/issue/task tracker, JIRA. Source control is a natural extension of those tools, which I'd guess have had some effort put into making sure they work well together.

Looks like Stash is what they sell to enterprises, versus BitBucket which is hosted. Anyone know if these are completely separate products, or if they share a common codebase?

I've found SourceTree to be a great GUI tool for working on Git/Mercurial repos.

All of which is to say, good on them. I'm glad to see two very respectable players in this space.