I have flown both AirTran and Frontier, between those two, it would have to be Frontier. Flew them twice from BMI-OMA-DEN. Great flight crew, the muffins were fabulous, older 737-200s but in excellent condition. Usually, you can get good fares without having a 14 day advanced purchase.
AirTran was good also, easy to upgrade to Business Class. The route I flew was BMI-MCO with a connection in Atlanta. DC9 from BMI to Atlanta and 737-200 to Orlando. The 737-200 was a mess, dirty and it looked old. I think AirTran is still trying to escape the "ghost" of ValueJet. The DC9 was pretty clean and did not look as old as the 737-200.
Just my thoughts!
DC-8 61/63/71 DC-9-30/50 MD-80/82/83 DC-10-10/30 MD-11 717 721/2 732/3/4/5/G/8/9 741/2/4 752 762/3 777 A306/319/20/33 AT