the simpsons is worn out, imo. south park just keeps getting funnier and funnier, and actually follows some sort of storyline, whereas the simpsons make up new things every episode, with the occasional follow up a few seasons later (vegas wives anyone?).
south park is in its 6th season, it keeps getting funnier and funnier, i loved seeing cartman make scott tenorman eat his parents, pure class. kenny finally died and i believe that was for the better.
it's a great show, trey parker and matt stone. are brilliant, if not a little childish... but there is alot of social commentary in the show, its great, and you need to be smart to catch it.