As far as I am concerned, GitHub is the king and queen of applications in the git world. But it has a downside that is not easy to work around: GitHub Enterprise is expensive. Keeping code on GitHub infrastructure is not always allowed by authorities and such, and then things get really expensive. That’s where, I think, Stash can come in.
Stash is a product of Atlassian, the same company that owns Jira, BitBucket, and a few other well-known developer tools. Given that Stash has only been launched this year, and judging by the screenshots, GitHub probably provides more functionality. But as I said earlier, GitHub’s price might be simply too high for some companies.
It’s also worth noting that both companies have recently received large investments (Atlassian got $60 million and GitHub got $100 million). Since private repositories and in-house installations seem to be the primary source of income for both of them, I’m seeing a revved up competition between the two in the nearest future.