I doubt you would see both seasons of decoration. The Christmas stuff goes up around Thanksgiving...resorts start a bit earlier than that.
As far as weather??? Dec is a crap shoot. I've been in early Dec for 10 years now. There have been times when I needed mittens at night, not to mention the jacket over the sweatshirt, a scarf, jeans and socks!! When it gets down to the low 50's in central Florida, it's cold!!!
Other times, it's been very pleasant...warm during the day (in the 60's-70's) and down to cooler temps at night when the sun goes down.
If you are looking for a warmer stay? I would go down the middle of November. The crowds should be pretty good then..just avoid Veterans Day! If you go any time from the 10th to the 25th or so, you should get good weather and lowish crowds.