While they are not equivalent, they serve to achieve similar goals, and it's fair to compare one to the other. Github may be better, but it's not the point. People will be using github to do the same stuff they used to do on SourceForge. Where's the innovation?

Sourceforge is to GitHub as Friendster is to Facebook.

Yes, they are kinda the same idea, but GitHub realized that the social element was key, and managed to create a community around open source the way that sourceforge never did (and, I would argue, didn't even know to do).