I've done both now too. Been a few years since we did late Sept (we typically go first half of Sept now) but you can't go wrong with either!
Late Sept, at least 4 years ago felt pretty crowded with our "Sept expectations", and crowds were prob close to when we went this past Dec (Dec 15th-20th).
What holiday do you enjoy more, Halloween or Christmas? I enjoy both but think Halloween is more fun, and I enjoyed the Halloween events more than the Xmas Party events overall, but both are worth going.
You absolutely can't beat the weather in Dec vs the Sept (each-day-requires-2-outfits) humidity
You really should do both if and when able, and we didn't regret either timeframe, but for us personally we were glad we did December and Xmas, but will more likely stick to Halloween and melt in Sept's humidity in the future