I think pretty much anyone who likes family guy will have liked the simpsons at one point and gone off it because of family guy. Or at least thats how i feel The Simpsons is watchable and often funny but family guy is really funny. Although both are getting pretty naf at the mo, American Dad has my interest at the moment.
The Simpsons always have been and always will be better. Family Guy was funnier before it was cancelled and brought back, and now it only has a few good moments per episode. Family guy has also ripped off a lot from the Simpsons (look up the clips on YouTube, you'll be surprised how many there are!). I'm not saying Family Guy isn't good, but The Simpsons are way better.