Airtran and Delta are pretty equal on cancelled flights - they hardly ever cancel. I fly out of WI and there is a lot of bad weather here. The only ones that consistently cancel at the drop of a hat are the Star Alliance ones - US Airways, United and Continental
In fact, I haven't been canceled out of here going to many destinations on Frontier, Airtran or Delta yet. And, last week was a ferocious wind storm that grounded my coleague's United flight and caused him to have to call and re-route his flight.
My Airtran flight left the gate early. Delta's were all on time that day, too.