I would agree with the other posters.
I rent from PHL frequently - used to be National, now with Avis (due to changing companies).
I have had good service from the Preferred desk, since I always have to check with them as my space is never preassigned.
I would also say, that if you do find a car in the exchange, they won't know how to process it when you leave, and you will also find yourself at the desk when you return since they didn't process it correctly to begin with.
Leave yourself some extra time.
The people that work at check-in seem to be very friendly.