As a person who has done both, more short trips recently mostly because of our circumstances, I definitely like my 8-9 night trips in late August or early September. Don’t get me wrong I am not turning down the short trips but I just feel I can really get away and relax on the longer trips!
I would go with a longer one in December for you ladies since you have done September so many times.
I love the heat at Disney, I am weird like that. I love early September and bring on the heat but I also love December and have gone the last 3 years in a row.
I bet I am not much help!