If I could afford it, I would do BC to GF. Christmas is crazy at WDW, so you will appreciate being able to take a quick monorail trip to MK during your GF days and walking/boating to MGM and Epcot during your BC days. Bus service is pretty good from BC to MK, but I'm not sure what it will be like from CBR during Christmas. You will have a day bed at GF (not sure about BC).