I have been in Oct., but that was when it was considered value season. Now it is regular season and it is alot more crowded than in the past from what has be discussed here. You have a PGA tour in Oct, the F&W festival, lots of conventions. It seems alot of people caught on to it being a slow time and now everyone goes and it isn't slow anymore!
I have gone the week before Thanksgiving 3 times and while it is getting alittle busier than our first Nov. trip, it is still a slow time. There really aren't any lines for attractions. If so, just use FP! We never needed to. The weather is perfect and I think out of 3 visits it only rained once for half a day. Also, you get to watch the transformation to the X-mas decor. Really beautiful. One tip if you go this week, make sure you leave before the weekend before Thanksgiving. The crowds start coming that weekend and you wouldn't believe the difference in!!
Sept. I have heard is slow but, still really hot and humid and also the chance of a tropical storm/hurricane and rain.
Try Nov., you'll be spoiled!! That week is also value season and we have always been lucky to get an AP rate or code. Not that that's always the case, but there's a good chance!