Oooooohhh tough 'un. I would personally go for knocked up as it has more of a story for me, and i think its funnier with funnier characters (leslie mann and paul rudd are really good togevva)
I really think both are overated to be honest. I find them funny, but not as funny as everybody makes out - I much prefer the likes of Old School and Step Brothers etc.
BUT, If I had to choose I'd go for Superbad, it's much sweeter.
Knocked Up for me, I thought Superbad was complete and utter shite and surprised Adventureland came from the same director(though I don't think he was involved in the terrible script for Superbad?)...
I say Knocked Up, I'd rather watch Pineapple Express again over the two.
Apart from Jersey Girl. Utter rubbish. And Chasing Amy's not that great either.
Back on topic, I'd go for Knocked Up, only because I've seen Superbad sooooo many times now, and the McLovin thing is starting to annoy me. Plus, I really fancy Seth Rogen in Knocked Up, maybe more so than Michael Cera in Superbad