This question is based on mere perception of where I usually tend to browse source code.
Bitbucket has unlimited private repos. In the case you wish to go public, obviously Github has a larger community, but it's also easy to just uncheck the 'private' box in Bitbucket.
So why has Github been the place to-go for hosting code?
[1]: https://en.wikipedia.org/wiki/Unix_philosophy, not that's objectively better, just that in this case it seems to provide an advantage.
[2]: When I signed up several years ago, I think it was only five private repositories...and Mercurial was favored over Git in the signup process. But free private repositories was why I chose Bitbucket over Github. I was using it for Coursera assignments.
[3]: Github versus Bitbucket may also be explained by Zawinski's Law: The yang to it's Unix philosophy yin. http://www.catb.org/jargon/html/Z/Zawinskis-Law.html