My friend and I had this convo a while ago, we both agreed that Family Guy had more LPM= Laughs per minute, while the Simpsons doesn't.
Family Guy was a little more edgier/goofier than the Simpsons, but it was basically the same show. Family Guy has more "flashbacks" , like when Peter remembers something; its a pretty fast paced show in how its edited. It also seems to have a lot more Pop culture references. Or at least ones that relate to young adults 18-25.
Simpsons is classic, a little slower, and sometimes deeper, i don't care about the new ones, the old ones can be as funny as Family Guy. Its a little more mature, with some political jokes, and i think with a show like the Simpsons it realates to bow high and low brow comedy.
Family Guy seems medium brow. :P
I love both of them, I just find my self laughing a lot more and harder at Family Guy :)