My experience has been that the end of Aug and the end of Sept are pretty different!
A few years ago the end of Aug was a relatively uncrowded time but has gained popularity recently. The lines are still short compared to Spring Break or Christmas but there are fewer walk ons than in the late 90s early 2000s.
I think parents are less likely to take their kids out of school right at the beginning of the year and that contributes to why late Sept is still relatively calm as far as crowds.
For weather, Sept has a slightly lower average rainfall than Aug and its more on the tail end of Hurricane season. August, it WILL rain for an hour or two EVERY DAY! There are a few exceptions, but they are just that....an exception. Every trip I've taken in Aug I've been faced with short, but often torrential, afternoon rain (approx 2 till 4)