This is based on second-hand knowledge:
My mom has been commuting MSP-MDW every few weeks over the past 2 years, and has flown Vanguard, Airtran, ATA, and NW. She always takes the first flight into MDW, and last flight back to MSP the same day. For on-time performance, i would rank them all close to the same (including NW) at about 75% on-time... but some of this is due to MDW's inherent problems of weather/only one runway and limited MDDW gate space. ATA seems to have the most trouble with lack of gate space as they juggle about 10 flights out of 8 gates at a given time, it seems. Thus, with some ATA delays, the aircraft is landed and sitting on the tarmac. Delays would be in the neighborhood of 30 minutes.

Delays on Vanguard were brutal: weather would push everything back an hour or two by the last flight of the night. It may be because Vanugard schedules flights with little recovery time to absorb delays. Service-wise, though, my mom liked service by the Vanguard staff most of the bunch, again including NW.

TravelRobb, you may want to break the airlines into hub-and-spoke carriers (AirTran, ATA, Frontier) and point-to-point carriers (Spirit, JetBlue) because as far as delays, passengers on point-to-point carriers may be annoyed, but they wouldn't be stranded in an intermediate city.

I would also consider the airport being served: Kansas City is much less prone to chronic delays than Chicago Midway, so passengers from LGA to LAX on ATA through Midway would have a different experience than on Vanguard through MCI. Frankly, in my mom's case, I feel better sending her on ATA because since they are MDW based, the pilots, etc. have more experience dealing with MDW's challenges-- much more than, say, ATL-based AirTran pilots.