AirTran is a low-cost carrier hubbed out of Atlanta and with a major presence in Orlando (I guess Orlando is considered a hub for them also). Customer service is okay -- I've never had any problems with them but I've read of people that have. Like Southwest, the only cabin service they offer are pretzels and soft drinks. Unlike Southwest, they offer assigned seating and a business class section. Their main selling point is that they've been replacing their fleet with brand new Boeing 717s, modeled after the MD80 series. They still have some older DC-9s but I don't know how many.
I've never flown Midwest Express, but I've heard they have very good service and the whole plane is first class seating.