Sourceforge was always hard for me to use, the standard template (code, forums, issues, mailing lists, wikis) were hard to set up (my experience) and use.

GitHub's UI really made impact on me, and the short name, the agenda from the people (I've seen couple of videos from the founders talking about not github, but GIT itself).

I still get very lost using git (p4 user at work, so svn feels closer) but with enough help from github I can see it done.

But all in all github is personal first.