I have most of my personal stuff on Github. For work, I use the company SVN server, which is what everyone else uses. A bit annoying, as it isn't as good as git, but it gets the job done.
GitHub wins it for me (I use it for some personal projects like my IRC bot framework, ChatterPy). I love the in-built issue tracking, and I find it useful that if I include 'Resolves #issue number' in the commit message, it automatically closes an issue, amongst other things.
Interesting! To those who prefer bitbucket, what makes it better than github? I think they both have decent interfaces, but there's obviously more to it than that.