the rainforest episode was one of the earlier ones, before southpark became a more of a social comentary.
And yes, Family guy has its satirical moments. But they come in the form of ramdom offshoots of the main plots, if you can call them plots. Very entertianing, yes. Good at getting a message across? Not so much. Southparks emphisis on social issues makes them much better at getting a message across to their audience.
Red hot catholic love, paris hilton, chickenpox, proper condom use, Cartmens increadible gift... Southpark is far more Didactic, while keeping the humor entact. Thats why I think its the better show.