We have done Mexico twice on Princess and once on RCL. I like both lines and think they both do a great job. When we took the children we went on RCL; when it was just adults, we went on Princess.
I really don't think you can go wrong with either one. I would probably go with the one that gets you the best cabin for similar money.
As others have stated, there will be a lot of children on both over Christmas.