I have stayed at both Beach Club and Boardwalk many times. For celebrating your milestone birthday in December, I would choose BWI.
At Christmastime, the BWI lobby is far better decorated, even tho the BC has the carousel. A lot more green swag, trees, and ornamentation at BWI.
Rooms at both Resorts are new. BWI is more colorful and thematic (but not in your face).
The interior common areas at BWI feature rich details like 1920s background music, palms, paintings, knick knacks, and old timey treasures. Beach Club hallways just don't.
Beach Club is marketed as the kid/family Resort around Crescent Lake. With the character meal off the lobby, the lobby is never peaceful but rather filled with noise and clutter.
The Beach Club is great, and I have always enjoyed my stays there, but I do give a slight edge to BWI.