We use BitBucket since we're a small shop from a developer count standpoint but we have a ton of small repos plus we migrated over a ton of legacy svn repos. GitHub gets expensive real quick when you are in that situation.
For anyone doing 'client work' rather than developing a single/few products, GitHub's pricing is almost prohibitively expensive.
We have a little over a hundred repositories. This puts is in the $200/mo plan for GitHub (125 repository limit).
Atlassian prices per-user. Our small three person dev team costs us nothing. We'll reach the next pricing tier when we hit our 6th developer, at which point it will cost us $10/mo and will remain that cost up to 10 developers.
BitBucket's top plan is $200/mo. That gets you unlimited repositories and users compared to GitHub's 125 repositories.
In the middle tier, GitHub charges $50/mo for 20 repositories whereas BitBucket charges $50/mo for 50 users.
If you have few repositories but many users, GitHub's pricing is advantageous. If you have many repositories but few users, BitBucket makes way more sense.
