Hello Terese! Welcome to the Disney Parks Moms Panel. Fall is my absolute favourite time to visit Walt Disney World. The weather is usually amazing - the humidity of summer is gone, and rain is minimal. I find September to be less busy than October, because of the
Epcot International, Food & Wine Festival, which this year runs from September 27 - Nov. 10, 2013. This event is hugely popular, and while definitely a great reason to visit Walt Disney World at that time, does tend to increase the crowds significantly over September. From recent years, Disney has offered a Free Dining promotion that usually runs through the end of September as well, so that may help to sway you in that direction. For me personally, the only down-side to visiting in September are the love-bugs - harmless but pesky insects that seem to invade the area.
I think that whichever month you pick, a fall vacation will bring you gorgeous weather, fun seasonal decorations and lighter-than-summer crowds. I would be happy to answer any more questions you may have - just ask! Have a magical day.
Kristen