I was banned from watching both from my parents (mom mostly)
since Family Guy came on later at night, i could catch it pretty regularly. If only South Park also had a later showing time.
Thus Family Guy is my pick.
Also South Park kept up with current events and we forget certain details so the episode loses its overall effect.
I totally understand. Growing up, my father would actually WATCH IT WITH ME. At first, my mother was hesitant, until she watched some herself. I was encouraged to watch South Park and I love my parents for it.