We have visited in both Sept and Oct. (as well as a few other times of the year.)
What we found was:
Sept had less crowds, but shorter park hours. There are quite a few rehabs, but we know that we will be back again next year, so we didn't stress about a few things being down. The payoff for the almost lack of wait times made it worth it.
Oct had a bit more crowds, but nothing like any peak season. the park hours can be a bit longer, depending on what convention in in the area. We got several "notes" left on our camper door informing us of park schedule changes that took place all of a sudden. We were flexable, so this wasn't a big deal, but if you were on a schedule that might screw you up a bit.
The weather in both of those months is up in the air. Sept is supposed to be worse with storms, but when we visited (in 2000) the weather was much dryer and warmer than our trip in Oct (in 2002).
We have discussed it and if we could pick any time of the year (reguardless of having to take children out of school or any other factor) we would pick any time in Sept everytime.
- lori