We usually go in Sept every year because of the lower resort prices, but if you're staying offsite then the resort costs are a non-factor for you. I will say that 2 years ago we went in early October and it was heavenly -- perfect! The sky was pure blue our entire trip, not a drop of rain, temps in the 80's. I often think back and wonder if every October is like that. However, the resorts are a lot cheaper in Sept, so unfortunately that is when we'll continue to go.....
A plus to going in October is the food and wine festival in Epcot. It's a lot of fun!