I agree with Chicago. I know that's where a lot of national conventions are held because it's the most central, easily accessible city. That said, when you run the numbers for one meeting, what are the total costs for the meeting? One day/night hotel in Chicago, plus flights may be twice the cost of staying somewhere in the south. Plus, there would be the cost of booking meeting space.
I'm not sure if you use Microsoft Outlook, but there's a way to send a message to meeting attendees to have them "vote" on which option would be the best for them. It's called "voting buttons."
I'd narrow it down to three major places and then do it democratically. I'd also consider a fourth option of doing it Seattle, if people are willing. You can leverage your network here to save on the total cost of the meeting, and see if you can get a block hotel deal to save the participants money. Seattle's beautiful in September and the flights on SW are cheap. This of course, depends on where the majority of your attendees are from.