We will be there in Sept, we go every year in that month, usually the week of LD. Not sure on which resort, it's too far out to decide yet but alot has to do with whatever deal we can get. We are onsite kind of folks and usually have an AP but it just depends on how many trips we have planned. Our AP's run out in Sept 12 and we have decided not to renew and will probably not be back until March 2013 or later so we will let them lapse for a while.
Since we go in Sept every year we have alot of experience with the crowds that time of year. Since free dining was introduced in the mid 2000's the parks are busier but it's still the slowest time of the year. We find making ADR's to be important and skipping the EMH parks to be important. Espically if you like to eat at traditional lunch/dinner times.
In Sept more than any other time we have travelled to WDW it seems the crowds really pack in for EMH, espically PM. Crowd level might be low overall but if 70% of the overall crowds are at MK for EMH you are better off going somewhere else. We don't see this huge difference any other time of the year, IMO it's a combination of value season brings people looking for more bang for thier buck (and skipping hoppers) and shorter park hours which push the EMH's earlier in the night. On the flip side AM EMH's seem to be less busy that time of year in our experience, no one wants to get out of bed for them espically if they have been at a PM EMH park the night before. We hit the AM EMH parks and hop away at lunch.
Speaking of hoppers, we find them to be neccessary in Sept. Shorter park hours usually mean AK closing around 5 and DHS often closing at 8 or 9 (9 now that F! is back everynight) and MK closes at 7 for
MNSSHP nights and 9 or 10 on regular nights so having Hoppers gives you options for the evening. It also helps you avoid those EMH crowds.