When you get right down to it, The Simpsons and Family Guy are just two very different shows that are funny for entirely different reasons.
Family Guy might as well be a sketch comedy show featuring the same recurring sketch characters. Every character exists purely for the sake of delivering a punchline. Dialogue is used as a jumping off point for cutaway jokes. Characters walk on screen just to do something funny or have something funny done to them. The characters can hardly be said to be characters at all.
The Simpsons is more of a traditional sitcom. Characters enter situations, learn from their mistakes, and go through some sort of brief journey (no matter how important or unimportant it is). It just so happens that these situations are accompanied by comedic lines, and the world the characters live in isn’t bound by realism.
These two worlds don’t really mesh all that well together. You can’t put Lisa Simpson in the same room as Meg Griffin, because Lisa is a person and Meg is a punchline.
That’s not to say one is inherently better than the other. They’re just two very different cartoons.
And quite frankly, I haven’t watched either one in years. Their best days are behind them.