The difference is that BitBucket pricing is based on the number of collaborators. Their pricing page makes it seem like you even have to pay to have more than 5 collaborators on a public repo, whereas with github you don't. If I'm reading it right, then Github is still really good for Open Source projects that intend to have more than a handful of collaborators. While BitBucket is a good place to stick your private repos where you have few (or no) contributors.