Not sure about the crowds but tend to think weekend before Christmas might be better -- most kids would get out of school that Friday, maybe not even till the next Tuesday, so it might be less crowded than Thanksgiving weekend, when everyone's out. The weekend between Christmas and New Year's will be the most crowded all year!
Another thing to consider -- the hotels and parks start putting up their Christmas decorations right around Thanksgiving but don't finish them till after, so if you wait till the weekend before Christmas, you'll get to see more. The Osborne lights are not being put up this year, however.