ya.. there were a few differences..
1.  simpsons works on so many levels, the more u think about a joke, the funnier it is.
2.  the simpsons writers are very smart [not to say that family guy's weren't] and because they are so good, they have a show that every demographic finds funny; kids, teens, adults.
3.  simpsons jokes are often very very smart, and this relates to the above points. they build on other episodes, and the more u watch, the more u laugh.
4.  family guy has a lot of 'cheap laughs' - this type of humour doesn't often have high replay value.  due to the intricacy of the simpsons jokes, u can see the same episode numerous times and find new things funny.
i might not actually have any idea what i'm talkion about here, but it sounds right?!.. soo, yes.