I don't understand the fascination with github. Bitbucket gives you unlimited private repos for free, no special requirements. It has wikis, issue tracking, and contributors.
There's also less direct reasons, like GitHub Inc having a really open culture and GitHubbers are really awesome.
It seems to me that Git has won the popularity war,and GitHub has benefitted from that. GH has also focused on the social networking side more.
As an academic, I'd recommend BitBucket, and pretty much every academic I know prefers it to GitHub due to the free academic accounts.
To students preparing for graduation, I'd recommend getting a portfolio together on GitHub, but also spending time on Mercurial - it is much nicer in some ways, and variety is good for the soul.
To me, if the task is to see the code, they seem essentially the same — a storages one can checkout from, with a nice-looking web interface to preview the contents.
As for collaboration, DVCS are distributed by their very definition. Sure, clicking on "fork" button on web is easier that "pull; remote add; push" series of commands, but I believe this is completely negligible threshold as compared to actual work.