Family Guy used to be funny to me, and in small doses, does have its moments. Unfortunately, the show drove me to sheer hatred of it with it's constant musical numbers, cutaway gags that made no sense (some weren't very funny at all), most of the characters being bigger and bigger assholes without some redeeming qualities, some utterly offensive humor in the sense they failed miserably at Crosses the Line Twice, and Seth McFarlance hijacking the show in later seasons to bitch about his political/social views .
American Dad isn't perfect, but it's not nearly as terrible in most regards. The musical numbers and cutaways gags are nearly nonexistent, most of the characters have at least one redeeming quality to them, most of the line crossing humor is less terrible at hitting the mark, and it has a lot less preachy BS hijacking the comedy.