The parks are EMPTY in Sept. It will still be hot, but no waits for the rides! Also, due to the fact the parks are not busy, Disney always runs really good deals in Sept. I think deals are still pretty good in October and not very crowded, but Sept is better for rock bottom pricing. There are lots of indoor opportunities, so if you can handle the heat, and plan your day rotating outdoor and indoor rides, you can have a blast.
As someone mentioned, the parks close earlier that time of year, but since the lines are minimum, you don't have to stay as long to do as much or more than during peak summer and holiday season. They also do not typically offer fireworks and light parades every night, as they do during peak season, so you have to plan certain nights for those. We actually like the fact the fireworks are earlier, as we can leave the parks earlier and spend more time in the pool or relaxing at the resort.
Actually,you can't go wrong with Sept or Oct, so enjoy whichever one you choose!