It's not dreary most of the time in Dec in my experience. It's just that when it rains, it seems to last most of the day & be cold, misty, rainy drear vs. the sudden fast & furious thundershowers of summer. I still think early Dec. is a marvelous time to go. It's going to be a total luck of the draw on the weather situation no matter what. Dec. really is quite beautiful with all the Holiday Decorations. I've been 3 times in early Dec.
I'm sorry if my post was too pessimistic. I didn't mean it to sound that bad. I thought the question was of the types of rain, which is worse. I do prefer other times of the year in WDW, weather wise. But I think everyone should see WDW at the Holidays at least once. It's really spectacular, even if you do get one of those rainy days.