Okay I'm not Shelly but can I put my .02 in on your trip choice??
I think the week after Thanksgiving typically will be a little more crowded due to the taping of the Christmas parade - they don't release the actual date until closer to the taping of it but it's usually around then. Not only that, you may end up seeing some pop warner crowds. Not sure though.
As far as
MVMCP- people will tell you that they liked
MNSSHP better. I agree. I didn't care for MVMCP, it was just sort of eh to me. The castle is beautiful all decked out in icicle lights, and the snow on Main Street is cool, but not WONDERFUL. You get cookies and watered down hot chocolate. Just.... eh.
But MNSSHP---- oh boy. The parade alone is worth the price of admission. Seeing the headless horseman come trotting down the parade route on top of that black horse with the sparks from his hooves flying behind him---- it is AMAZING. And then the grave digger with the real blood hound-- it is just simply the best thing ever.
The characters come up to a lot of people, more so than usual. Captain Barbosa walked up and kissed my hand. Cruella de Vil made my DD (dressed as Alice in Wonderland) curtsey to the Queen of Hearts and say 'Yes your Majesty'.
It's just really a special party. AND--- you get to trick or treat in the MK!!!! The villians are out and the characters are usually in a really jovial mood. My youngest DD, dressed as Tinkerbell, played hide and go seek with Captain Hook at his meet and greet. Just special.
Weather wise, Not gonna lie, Sept in FL can be horrific. BUT --- there are plenty of opportunities to go into shops and rides and restaurants. You take breaks during the day and hang out in the pool or take a nap in the cool and head back to the parks for the night. easy peasy! Our trip this past Sept, I only really noticed the heat one day and it was when we did Epcot. We arrived right before lunch and stayed thru the hottest part of the day. Brutal. One drawback is the risk of a hurricane coming thru and it does rain every afternoon. We lucked out and had no days that were complete losses. The rain cools everything off and sends the people scurrying for the exits. We wore ponchos and kept going.
In Nov/Dec you have the chance for rain, but also cool weather. You may not be able to stand the heat so this could be huge for you. It wasn't a plus for me, I hate any temperature below 75 degrees. I wore jeans just about everyday, except for one night at the MK when it was so stinkin hot I had to go buy a tshirt and change bc I was miserable. You do have to pack for everything during this time. I had packed the gamut from hats and scarves and jeans and sweats to shorts and short sleeves. The kids only swam once during our time in early Dec. In Sept they were in the pool daily.
Park crowds- the earlier you can go in Sept the better, esp if you can go before F&W. Waits this past Sept (1st-9th) were non existent. I have pictures of me and the kids in AK at 1 pm with no one in them. Same with Epcot. I was texting pics to Shelly so she can attest to it. I was astounded. Waits for Soarin were 20 minutes until the afternoon when it climbed to about 40 minutes. The CM's were standing around looking at the wait board and commenting about it one morning around 11 because the wait for Soarin and TT were both said 10 minutes. Crazy. In the 9 days we were there we only pulled one FP, and the return time was for 20 minutes later so we just skipped it bc we were on the way to eat lunch at San Angel Inn.
December, the crowd will be more moderate. It has become a well known 'secret' that the week after Thanksgiving is a lesser time crowd wise to travel to WDW, and therefore more people have started going.
Okay and also---- last but not least-- it sounds trivial but it was big to me... I got so tired of hearing Christmas carols in WDW!! Even in Epcot where they should have different music for the different worlds and stuff-- it was Christmas music. I was thoroughly tired of it when we left.
Hope that helped. It's always so hard to make a decision!! I'm hoping to make it back in Sept next year for the MNSSHP again.
Belle's had butterflies and fire on it. You know that is right up her alley, I didn't get to offer any suggestions at all on it.
Wow your gas price is already good! 
Ours is still around $3.40. 
I hate it when life and work gets in the way of my Dis time. 
I know that's hard to believe! Just from the time I've been knowing you, it is hard for me to believe, too. 
When I first started reading your TR's, he was just a little thing!
