Wow, this vote is an embarrassment to the cafe's intelligence. Besides having humor that is actually related to the plot of the show, South Park addresses major political and social questions weekly. You gain another perspective on many issues by watching South Park, which is a reason, even despite the language, that I wouldn't have a problem with my (prospective) child watching South Park.
Besides each episode being chalk full of meaning, the South Park characters are much more developed than their Family Guy counterparts, leading the interraction between characters being much more entertaining in the former. While I do enjoy the relationship between Stewie and Brian in Family Guy, the Cartman/Kyle relationship is just as entertaining and is viewable much more often than Stewie and Brian's. I also tend to see the South Park secondary characters (Butters, Token, Towelie, Mr. Marsh, etc.) play much more prominent roles in specific episodes, and succede in doing so...that can't be said as frequently about Family Guy.
Look, I own multiple seasons of DVDs for both shows. They are both funny and are certainly among television's funniest shows. That said, saying Family Guy is better than South Park is nothing short of wrong.