September might be a little less expensive for airfare, depending on where you are flying in from.
The shoulder months of May and Sept tend to be pretty good for that.
I like late August into the fall to go to Hawaii because I get chilled very easily in the ocean and although Hawaii is a fairly temperate climate, I have found the ocean is warmer during the late summer and most of the fall. I love to snorkel so this is a big deal for me.
Because kids are back to school and the Hawaii Oct school breaks dont usually start untill Oct September is a very sweet spot as far as crowds at Aulani go. This is true for most of the touristy spots as well so it is a total win-win.