There is usually some sort of fall promotion at Aulani, but renting DVC points is usually a better deal.
Some of the pros for visiting in September are smaller crowds, less staycationers, and lots of sunshine. It's a hot and muggy month, but still much more pleasant than WDW in the summer. The major con is that September is hurricane season. While the odds of a hurricane actually hitting are low, you'll find that most businesses will shut down for 2-4 days if the hurricane path is anywhere near the islands. If you're only here for a week, that could be half your vacation.