I used to like both equally, but now I think both are past there prime. The Family Guy episodes are starting to make no sense, (not like they did before, but just more so now) and I think they're definitely running out of ideas. And as for South Park I think they're just getting annoying, and the episodes are just not funny at all.
South park is completely stupid. I don't think that 12 year old going around talking about porn and swearing is funny at all. Family guy is one of the funniest shows ever created
Family Guy is by far better. I've watched some South Park and never found it all that funny. It has its moments, but Family Guy is definitely funnier and all around better.