South Park by a mile. B&B was humor on the intelligence level of 4th graders. B&B is nothing more than the same jokes about bodily functions (and slang names of body parts) over and over. ("he said stiff. huh huh huh huh)
In addition to being just as funny (funnier, actually), as others above have said, South Park is actually a commentary on society. So, we're laughing at society at the same time we're laughing at what the characters do. I can't think of any other shows (other than news shows) that have been so up to date on the issues at the forefront of society, yet show us how ridiculous society has gone with their concerns. i.e. child molesting priests, gay scout leaders, SARS "Stan, I'm sorry, but I have SARS and there's only a 98% chance that I'll live," war in Iraq, metrosexuals, even downloading of MP3's
I suspect this South Park craze has more to do with the average age of the voters here, rather than it being any better than Beavis & Butthead.
I'll bet it has more to do with average intelligence