I look at it this way.
Southpark is like an edgy political comedian.
Half the time he's really hilarious and makes a god point... the other half of the time he's so offensive you have to turn off your tv... even if he has a good point.
While Family Guy is like Eddie Murphy.
Really funny for a while... but now it just hurts you to see him do anything...
Ever since Family guy came back they just stupified everything, and intensified and distilled a lot about itself... to the point of where it's just bad.