I have not used Priceline or Hotwire, but I have used Travelzoo (3 times) and
www.hotelkingdom.com. I have no issues with either website and would use them again (obviously, as I have used travelzoo 3 times!). With travel zoo I usually book one of the weekly Top 20 deals. Register with them and you will get the Top 20 emailed to you every week. You will certainly find something for your Sept dates, although right now I think they are mostly spring & summer specials.
What I like about Travel Zoo and Hotel Kingdom is that you choose the hotel you want. With Priceline you choose the area, but don't find out which hotel it is until after you buy. Travel Zoo (usually) just takes your credit card to hold the reservation, but nothing gets charged until you check in. The hotels we stayed at with them had a 4:00 or 6:00pm same day cancellation policy, so if something happened and we were delayed or had to cancel we could.
We never stay onsite. We have stayed in both the Lake Buena Vista and Kissimee areas. I would recommend a DTD or Lake Buena Vista area hotel for you for one night.