It's a matter of personal taste. For me...
I prefer PHL over CLT for a number of reasons:
1. If you do have time to go into the city during your layover there is MUCH more to do and better dining options in Center City Philly that there are in Charlotte. Cab ride into Philly is flat fare (into Center City) and takes about 15 min. CLT takes a bit longer.
2. PHL has a bit more to do in the airport if you have a layover and not enough time to leave the airport. I agree that the clubs are a wash.
3. IF there is a travel disruption (and there usually is in PHL) you have a couple more options to get you across the pond from PHL than you do CLT. There are limited TATL flights out of CLT so if you do get stuck you could get stuck for the night. PHL has a couple of flights to LHR and other larger European hubs where you could connect and ultimately find your way to MAD.
4. Clearing immigration at PHL varies. In the afternoon while the TATL flights are all coming in it can be a bit hectic. As others have said, Global Entry is the key. Well worth the money spent.
5. Whatever you do...avoid the TATL 757s on US. THEY ARE AWFUL, HORRIBLE, DISGUSTING, LOUSY aircraft. Flew one last month to BRU and hated every minute of it. Seats do not recline much, service is lousy, planes are falling apart. The IFE is on tablets which just don't fit anywhere if you have a laptop out or your meal.
As an FYI, AA flies 777s from MIA-MAD so if you don't mind going a bit out of your way and can make the connection work that may be a better option as the new 777s are lovely.