We did both Halloween and Christmas last year and Halloween was by far the better for us. The atmosphere was amazing..particularly on the Halloween Party night which was the best Halloween night I have spent anywhere (including the USA)...excellent entertainment etc, and I really could not fault it. We are booking to go again this year as it really was THAT good!
On the other hand, having experienced such a fantastic trip at Halloween, in comparison the Christmas trip was a let down...much more crowded and really freezing (which may have impacted on our moods a little as we are not winter people), and the Christmas atmosphere was very over-rated. There were a couple of nice shows and that was about it. Far less characters wandering round (probably due to the cold), and we so missed the interactions which we had at Halloween with the characters and CMs in general. The overall atmosphere at Christmas seemed far from festive and it was more endurance than enjoyment.(again because of the cold, queueing in the cold is no fun at all!)
However, this is just our opinion, and I am sure if you have young children they would find it magical, so don't be put off on the account of my opinions.....there are a lot of people who love DLP at Christmas as I am sure you have realised.