Miami actually has more USAirways flights than Ft. Lauderdale, but that's because Miami also has USAirways Express.
Ft. Lauderdale International Airport is to Miami International Airport what LaGuardia is to JFK. FLL is the #1 domestic gateway in the Miami area, and has more frequencies to most of the major markets. In fact, starting in October, FLL will have 7 LAX flights (1 DL, 2 B6, 4 AA) compared to MIA's 6 (1 UA, 5 AA). Miami is the international gateway, Ft. Lauderdale the domestic gateway. Also at MIA, AA competes with USAirways too all three US hubs. At FLL, only airTran competes with FLL-PHL service.
Though you will see USAirways grow in Miami in the near future. Miami is a Star Alliance Interantional Hub and USAirways will play a key role in the new Star Alliance Terminal now under construction in Miami. It is an $856M terminal that is the first of it's kind: it will combine all alliance operations under one system. From check-in to loading the luggage to catering to gates, all 15 gates at the new terminal (all interantional, all can handle a loaded 747-400) will be shared by the Star carriers at Miami, which will turn Miami mainly into a US/Europe>>Latin America/Caribbean hub. USAirways may very well play a key role in the Caribbean part, even though this is AA's territory.