I have been to both Buckingham Palace and Windsor Castle in the past two years, both times around the Christmas holidays.
The Buckingham Palace tour during the holidays season is small, less than 30 people total with guides that work at the Palace. The quality of the guides brought the workings of the palace to life. In addition, we happened to be there right after the King hosted a state dinner, so we were able to see the remnants of the dinner and learn about the protocol. The holiday party was the next night, so the guide shared stories about past holiday parties. Because the tour was small, we were not rushed and had ample time to explore the excellent art collection.
Despite the quality of the tour, I preferred Windsor Castle (which we did not have a guide). St. George’s chapel is interesting and the people that work at the chapel have a wealth of information about the history and the community. It is well worth your time to stop and chat to them while you are visiting. The castle itself is more vibrant and the doll house is fun. The grounds are also quite nice. During the Christmas holidays, it is also very nicely decorated while Buckingham Palace was more austere.
I am going back this December with my niece and I am going to take her to Windsor Castle. However, if you prefer a small tour with excellent guides, Buckingham Palace was excellent.