See you there! We'll be heading there those same dates (flying in the 2nd most likely but not going near a park until the 3rd). Having done those same dates in 2013, last week of September in 2016, and end September / 1st week of October in 2017, it's definitely our favorite time of year to go.
Crowds get a little more in October than September, but not bad at all. I've found crowd levels are still affected by things like Extra Magic Hours, Halloween party days, etc., but even with the zoo that evening EMH at Magic Kingdom was last year, we still got a lot of walk-on rides in after HEA and fireworks. Halloween party days are nice, both for the daytime (or 4pm entry if you're only using a party ticket) and during the actual party (as long as it's not the first party or end of October closer to Halloween).