I believe Mac_TLC (not sure of his exact user name) has a website with a comparison of the lines. If you do a search for his name and look at his posts, he answers a lot of posts about RCCL, one of them should have the link to his website.
I have sailed RCCL a couple of times and it is actually my favorite cruiseline, however we have not taken our children on it yet, so I cannot compare the children's programs. When we sailed
DCL or DS's were 5, 7 and 9.
From an adult perspective I can compare the cruise lines for you. Remember though, this is my personal opinions, and everyone's opinion are different:
Ships
RCCL - Voyager class, 4 story promenade, very open and spacious, you would never know you were on a ship, as well as the centrums at both ends are at least 8 or 9 stories tall. Dining room is three stories and open in the middle, very opulent, with chandeliers etc.
Radiance class ship - Most beautiful ship I have every been on, glass everywhere where you can see the outside, even from the elevators, 9 or 10 story atrium in center of ship. Two story very grande dining room, large beautiful theatre with balcony etc. Very beautiful indoor pool area (great for Alaska).
Artwork on both types of ships is amazing, sculputes in the stairwells, paintings in public spaces, etc. Very beautiful, and all different styles, something for everyone.
After sailing both RCCL ships, I felt claustrophobic on DCL, with low ceilings in the dining rooms and smaller more closed in theatre, and I missed the variety of artwork (only Disney artwork)
Pools RCCL - salt water, however bigger pools than DCL and more room to swim
Food - I thought the variety and taste was better on rCCL, but that is very subjective, my husband thought DCL was better.
Variety of snacks - DCL has food up on deck, however they shut down scoops in the evening so you couldn't get an ice cream after dinner, and the only other food that was available was the pizza in the evenings. (we had early seating and sometimes around 10 or so we would want a snack, and you couldn't get a hamburger or anything, just pizza unless you ordered from room service)
RCCL - Voyager class ships have food available 24 hours on the promenade, sandwiches, pizza and ice cream and cookies (as far as I can remember, they may have other stuff. Also have Johnny Rockets (nothing can beat that! although not sure of their hours)
Show - Can't beat Disney for their production quality here.
RCCL - nice variety of singers and comedians, jugglers etc. It really depends on the week who you get, they change it up a lot, but maybe not so kid friendly (although I wasn't thinking about that since my kids weren't with us). The ice show is amazing and a must not miss!
RCCL - Cabins are well laid out, but most are geared toward 2 people. I've never seen a cabin made up for 4 so I can't tell you how it is. Plenty of storage space, and the size is comperable in living space to DCL since the extra sq. ft. in the DCL cabins is taken up by the 2nd bathroom. TV is bigger than DCL and I enjoyed the RCCL TV which lets you check your shipboard account, order room service, book excursions etc all from your room TV.
Service - I would rank them equal on service, both were EXCELLENT!! ( My favorite part of cruising is the service, I loved being catered too!!)
Kids clubs - This is very subjective, and since I didn't take my kids on RCCL yet, I can't give my opinion, but I can give you some facts that I have read about.
RCCL - no pagers (I have heard they have a few for the very little ones). Have sessions , open for a few hours in the morning, then they close for lunch and the kids must be picked up. Same thing in the afternoon then they close for dinner, then open again in the evening. After 10pm there is an hourly charge for the clubs. Offer dinner only a few nights during a cruise where your kids can eat with the club, but must be signed up for in advance, only so many openings.
I have heard people report that their kids liked RCCL better than DCL and vice versa. A lot of it depends on the child, and there is not really a way to predict it.
I hope this helps, I tried to hit the highlights. I love cruising, and I don't think you can go wrong with either trip, both would be wonderful! And, since you are going at christmas I would think DCL would do christmas up just a bit more than RCCL, which would probably sway me toward DCL for christmas!
We have decided that hte next time we sail with the kids we will sail RCCL, Even on DCL they didn't spend more than 2 or 3 hours a day in the clubs, which left a lot of time with us. Therefore I think they will enjoy the ice skating, rock climbing, miniature golf inline skating etc. Also, for a family of 5 the cost is cheaper on RCCL by 1-2 thousand dollars on a holiday week, that's a lot of money!
If you have any other questions feel free to PM me.