SF is not "social", quite slow and is more difficult to use for developers. However, I feel that it is more end-user friendly. Maybe not inherently, but because of the differences in culture. There are tons of software there that I can just download and install without any tinkering. I don't recall a single application that I've ever installed from GitHub.
I think for most software on Github, the "end users" are themselves developers. Libraries, vimscripts, languages, programmer oriented utilities, etc.
I almost never got that sort of software from SourceForge. Most things that I have gotten from SourceForge are more "user application"-y. I can't think of any examples off the top of my head since it's been so long.. maybe things like Wesnoth.
Basically what I think I am saying is it depends on who the end user is.
