Well, I think that South Park, offensive and sick as it can be, has more heart and more of an actual message than Family Guy. Family Guy has a good formula, the runaway gags are undoubtedly the best part of the show. But a lot of the newer episodes are just cynical and the jokes are too low even for them. And I loved how Quagmire told Brian off, he deserved it, I know it's just a cartoon, but Brian had been pissing me off for the last few seasons.
I used to loved Brian, but now, he's just a hypocritical overblown liberal jerk.
And I used to love Stewie, but he's not evil anymore,he's just a little John Waters.
With South Park I have my nitpicks with too, but I'd pick it over Family Guy any day.