I belong to the US Airways Club, and I have to say that it's not an issue of which airline has the best club, but rather which club at which airport.
For example, the US Airways Club in Boston is fantastic. The one in Philly is great too. But the three in PIT leave something to be desired.
As a member of US Airways, I can also use the American Admirals Clubs if I'm flying either airline that day. I've used the AAC in DEN, ORD, JFK, and PHL. All were relatively nice.