To add on to the "best bang for your buck part". Following some simple logic, the DVC point system for a room is based on a point system. I believe there are 5 different point tiers per year.
The lowest dates for DVC points for rooms are all of Sept. and all of Jan. and Dec. 1 - 14, this along with Disney offering
free dining in Sept. most likely provides evidence that these are the months they will offer the best deals. Also remember with Disney, you can book any vacation for a fully refundable deposit of $200.00 as long as you cancel at least 31 days prior to the arrival day. You can also change the reservation if you find a better offer throughout the year. Disney is very accommodating that way.
May is in the 3rd tier for points on the DVC dates, so it's not the worst deal, but not the best either most likely.
We go just about every Sept. and we also went in May this year as well. I will have say that I landed a great deal early in the year when WDW announced "Free Dining", not this second one they just announced.
Both months are about the same heat factor, however there are a lot more toddlers and families running around in May. They also have cheerleading events every May so you have to watch out for those as well. Flower and Garden is going on in May and it's beautiful at EPCOT if you are into F&G's like my wife is. Crowds were on the low side of moderate I found, but not nearly as low as Sept. crowds.
Sept. it's still hot, but far less toddlers around. (The heat factor and the first month back in school along with the higher threat of Hurricanes is why crowds are low I'm sure.) They have Mickey's Not So Scary Halloween nights at MK in Sept. which are highly recommended. EPCOT has Food and Wine going on and the parks are usually around there lowest crowd levels of the entire year.
I've never been in Jan. or Dec. so I have no information about that time of year, although obviously it's much cooler and Christmas in the parks should be going on.
Maybe someone who has been at that time of year can chime in?
Another bang for your buck that is worth looking into, is Staying at one of the "Disney Springs Hotels". They offer the same 60 day FP window and Extra Magic Hours that staying on property offers and, often times less than the value on site resorts. Of course, you are not eligible for "Free Dining". I'm not sure how all the buses work to the Parks from those hotels however, but you can simply call them and find out. I know Disney Springs buses only travel back to the resorts, but depending on your budget, it may be worth the little extra time spent.