Our last two trips have been in September. We were in a 2bd at BCV (our home resort) in the last week of Sept 2008. Then we stayed in a studio at BLT right after labor day. We checked in on Labor day and left on Sat. BLT was pretty empty as far as we could tell (pool, community hall, bus stops) were all uncrowded. I agree that any of the dates would be comparable. We did not book at 7 months for BLT because the resort wasn't open until August of 2009 and there was a special day for non-home resort bookings. I called early that day and got a standard view studio without waitlisting.
A couple of things to think about...The weather should get less hot as the month goes on. (Obvious, I know!) The labor day week was not too bad, but we live in TN and are used to 90 degree weather. You also might want to try the Food and Wine Festival that starts at the end of September/early Oct. We have enjoyed it during our October stays. Good luck in whatever you decide and I know you will have a blast. We are planning on moving our Oct 2010 reservation to BLT if we can because we loved the resort.