I always choose the one that I can use Disney resort luggage checkin. I believe United is the only one from your list. If this is am important factor for you, that should help. We have used the luggage service at WDW for the past 3 trips and it works great. You just drop off your luggage at your resort and they send it to the airport for you. you don't have to be using DME to use this service, plus they print your boarding passes. Only certain carriers will let you do this. I know Southwest is NOT one of the carriers, but not sure about Airtran.