Hi, we have used both airlines, and I have to say that we have never had a problem with Spirit out of AC! I like the airport and the parking is a lot easier then Phila. The one advantage with US Air is the number of flights, last time we flew Spirit I believe they only had two flights a day from AC. My daughter is flying down in November, she took USAir cause of the flight times.
By the way we also live in Marlton, nice to hear from a neighbor!