Christmas eve and Christmas day should be fairly light traffic. Sure there are people running about going to relative's houses and all, but no where near the amount of people driving home on a workday. Besides, the Conquest sails Saturday to Saturday.. which would be light traffic anyway.
It really depends on where you want to spend the holiday. You will most likely be off the ship by 9AM or so, at a 12 hour drive you could be home by 9 PM and have Christmas at home.
In the end it really boils down to your wishes.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007