I preferred Knocked Up because the writing was better, the characters were better, I laughed more and it had a better cast. Though that makes it sound like I hate Superbad, I really don't. I just prefer Knocked Up. Superbad gave me a lot of chuckles but it was never laugh out loud, tears in my eyes funny whereas there were several scenes in Knocked Up that were for me. Superbad's humour and setting was a bit one dimensional, and I'm totally fine with that; I have no problem with high school comedies, I just felt that Knocked Up offered more. Especially cos I'm a fan of Apatows TV shows 'Freaks and Geeks' and 'Undeclared' (Check them out) and it's nice to see his regulars pop up.
Anyway, if you liked em both and the general Apatow/Rogen crew then check out the trailer for Pineapple Express.