I have flown both, and personally, I prefer Air Tran out of PIT (Southwest not an option yet for PIT). Air Tran planes are in excellent condition, and I have found their employees to be more, well personable. I have had good experiences with US Air's gate/check-in employees, however the flight staff just seem rude to me. However, US AIR usually offers better times, and sometimes matches AIR TRAN's rates. For our trip, I chose a one way USAIR flight down, and an AIR TRAN flight back. If all things (mainly cost and times) were equal though, I would go with AIR TRAN. It's sad how a few rude employees can turn you against a whole airline, but it's true, especially when there is competition out there who treat you right.
And as far as USAIR not charging fees, I fly them for business related trips often, and get charged the change fee everytime a change is made. Don't know about the rate decreasing, because every time I check a flight, the rate has gone up and not down.