I own Family Guy, and I've owned TSPP. I actually had them side by side at one point. I love TSPP quite a bit, and it got sold for all the wrong reasons (I got a very good price for it). However, I don't think I'd ever sell the Family Guy. They made a lot more TSPPs. If you come accross a decent Family Guy at a good price and you're on the fence, I'd grab it.
Family Guy is a good game, not great. Take the theme away, I wouldn't own it. However, TONS of voice clips, excellent theme integration, mini wizard modes, fun modes, good flow, excellent toy, it has a lot going for it.
TSPP, excellent rule set, good theme integration (but not as good as family guy). Zero, absolutely the worst flow game I've ever played. If they could have done just one thing, not hold the ball up at the Springfield Elementary shot, I think it would have done wonders for the flow.
Both great games for different reasons, I'd like to own TSPP again some day, and I think I will, but Family Guy overall for me is the better package. Personally, I bought LOTR. I hate the theme, but it has a great rule set with damn good flow. I wish the LOTR game was the Simpsons theme, I think I'd have my ideal pinball.