We went the last week in August and over Christmas(12/20-29) last year, so you would think I have a good answer for this question. The problem is Im thinking its a toss up.
Heres my take and see what you like better...
Aug- Hot(I prefer this), Dec chilly and at times, downright COLD! This also made packing easier in Aug as opposed to Dec
Aug- Needed to go swimming everyday, Dec swam once. This gave us more park time in Dec, but we never got to use the Poly pool in 5 days there which was a bummer.
Aug- Fairly low crowds, minimal waits, Dec-by doing EE and FPs we avoided a lot of busy times, but it was definitely more crowded after Christmas and we saw some pretty long waits.
Aug- Youre in Disney, Dec youre in Disney for CHRISTMAS...with a lot of extra magical touches.
Thats all I can come up with for now, but Ill keep thinking.