Family Guy is pure humor, and that humor isn't even that good. It has no story, very plain, one-sided, "unrealistic" characters, and some of the jokes fall embarrasingly flat. Don't get me wrong, it's a funny show, but it relies way too much on toilet humor , and the "let's make fun of the American family" spin has been beaten to death by The Simpsons. It's good for a few laughs, but nothing more.
Futurama, on the other hand, is a classic. The humor is very sharp, and the show is hillarious on many levels. It pays great attention to details and is filled with subleties that make it more "complete". Aside from the humor, the show has very dynamic, "believable" characters, brilliant writing, amazing animation, and everything is seamlessly integrated with the humor. It's one of the few cartoons where the characters and storylines can actually get to you. It can be satyrical, touching, thought-provoking, and always entertaining.
Family Guy lacks so much of what Futurama has, and the things that they have in common are done better in Futurama. There is simply no comparison.