Frankly I want to vote "both" for very different reasons. I am a very VERY Christmas person. As in before thanksgiving I'm itching to get my house transformed (literally) for the holidays. It's my favorite holiday. I'm not really big into Halloween
I love visiting WDW at Halloween / it's fun, the atmosphere is very festive, I love love LOVE the Halloween party. However depending I what time you go, it's still very hot and humid (which is really the only thing i DONT like about visiting at Halloween. The crowds are lighter in late august, and September. The crowds do build up again as you get closer to Halloween, but it's still relatively "lighter" then other major holidays
I love visiting at Christmas just because of how festive everything is. I enjoyed the Christmas party and everything is beautiful. During the beginning of December the crowds are pretty light, but as you get closer to actual Christmas, the crowds get huge. As a plus side, the weather is a lot less humid then day in July or August. We have had to wear a light jacket in December and also bathing suits. So the weather is a little more unpredictable.
Neither is a bad choice - if you don't have kids school schedules to work around go after thanksgiving and before Christmas (as early as you can in December). The weather is awesome, crowds are very manageable and the decorations are Devine