As an Orlando resident, I am pulling for Orlando to win out here.
The biggest strike against Orlando is obviously the weather. It can be absolutely gorgeous for a marathon, but is at least equally as likely to be warm and humid.
On the positive side, it is definitely going to be a flat course, we have one of the best event management/race timing groups in the country here (they did the timing for the trials in LA, do all of the Disney races here and in California, as well as lots of other national caliber events),
have a great airport with plenty of cheap direct flights, ample hotel accommodations and we have the theme parks as a bonus (I get that most of us don't find them appealing, but they are a draw nonetheless). And there is a very lively bar, and surprisingly well developed restaurant scene in and around downtown.
Plus, I really think the city would turn out and support the event. The city has done a great job supporting the two bowls that it has at the end of college football season, and has been impressive and how it has supported the Orlandos City Soccer Club. I think you would have streets pretty well-lined with spectators if you made it a 10K course through the core of the downtown area.