We almost always go in September. We love it. We have noticed over the past few years that there really isn't a "low" period anymore, but September is definitely better than say July or April. As for the resort, I am emphatically in favor of staying at Art of Animation over the All Stars. The resort features are much nicer, as are the rooms. Plus, you have your own buses at AoA where as all three All Stars share buses, something that ended up driving us crazy when we stayed there.
Another great reason to visit in September - you get the added bonus of the Halloween parties. An event we adore. Have fun! I think September is a great time to visit.