I think it depends on what you are hoping to accomplish during your stay at the parks. Early December this year has been more crowded than in previous years. That could be due to people who have rescheduled due to the hurricanes and the 50th anniversary celebration. It could also be the secret is out and people want to enjoy the Christmas decorations and are trying to avoid crowds and are willing to pull their kids out of school or whatever else it takes to accomplish that! The days directly before your January 27 sail date would be much less crowded but no beautiful decorations to enjoy.
My thoughts are if you want to go at Christmas, enjoy the decorations, tour the resorts, enjoy nice meals, maybe go to Mickey's Very Merry Christmas Party and don't care if its more crowded and you don't get to ride as much, then go in December by all means. Early December is still less crowded than Christmas week or July.
However, if you want to commando ride as much as possible and hate crowds of any kind, then that latter part of January would be better. Hours will be shorter and they may be refurbing some rides due to the slower time of year but there is something to be said for nice, uncrowded parks! Good luck with whatever you decide
