American Dad > Family Guy
American Dad has a better, more diverse family dynamic. Chris from FG is just plain unlikeable. There's nothing remotely human about him. Peter had his moments in seasons 2-7, and Brian was fantastic until they literally killed him. Since then he's been a completely dry political joke for the last like 2 seasons straight. It's getting obnoxious. Stewie had like 1 good line in Season 1 and has slowly turned from evil genius to gay stereotype since then.
Meanwhile Haley has a more colorful political tone than Brian that is much more open to comedic possibility, and while Stan is pretty much just Peter (Voice and all) but has a more interesting dynamic outside of the home that completely contrasts with both Haley''s extreme liberalism and Steve's dorky nerd attire.
Francine is about even with Lois all things considered. Housewives in these animated family sitcoms are pretty much always the same - they stick with their dumbass husband for no reason whatsoever and provide a voice of reason when the episode's focus gets off-course. They pretty much both just hammer in the sensible moral dilemma.
Roger is a bit much, but his one-liners can be pretty good. I don't mind him. Klaus is probably the lowest point of the cast, seeing as he literally cannot do interesting things. He is stuck in a fishbowl.
As for side characters, honestly I think family guy has them beat. Most of the recurring characters outside of Steve's friends and Haley's boyfriends are very, very forgettable. Whereas Family Guy can - and has attempted to - make it's own spin-off with it's side characters. Cleveland, Quagmire and Joe are very mentionable, with Mort Goldman and Adam West having their own moments. Even some of the short-lived characters have been lasting impressions on the series, like Julian (Brian's dumb girlfriend) and Not Cleveland.
Family Guy has some really, really dumb running gags now. The knee thing was so stupid that they actually made a joke about how overused it got before putting an end to it. Fighting the chicken has happened so many times that you could pretty much make a double-episode out of it now.
Overall, I prefer American Dad. It's got better main characters and has less filler gags. Seth MacFarlene has expressed that he no longer likes to work with the Family Guy series, and has said that "primetime shows should not go past the 7th or 8th season." I mean he killed off Brian one time just so he could not spend so much time with it.
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