[MitchFam]

I would probably pick Christmas. I just can't do hot AND crowded. I would following the Touring Plans in the Unofficial Guide to Disney book (~$20 at the bookstore or online at touringplans.com) and try and beat the crowds early in the morning. Otherwise the parks may be full before you even get there! I haven't actually used them since we always go off season (1st and 2nd week of December are terrific!), but I would surely give the touring plans a try. At least then you might have a fighting chance.

The Christmas decorations are magical. You MUST see the Osborne Family Lights at DS. Most magical part of the whole trip, IMHO.

Also, at touringplans.com, they have a "crowd predictor" for each day...maybe that would help you decide when to leave or return. We always use it when we plan...and end up helping friends pick, too!