I haven't looked at the pricing for BitBucket but it seems (in my experience) you can have as many contributors as you want in an "organization" on GitHub and you only pay based on the number of private repos that organization has and forks of those repos don't count.
We currently have 9 people accessing our 12 source repositories (9 BitBucket users or 9 GitHub collaborators). With BitBucket we pay 10USD/month, with GitHub we would have to pay either 22USD/month (the Medium plan) or 25USD/month (the Bronze plan).
In the end, BitBucket's pricing structure is much simpler for us and fits our model much better.
Is that not your experience?