If you don't plan on flying out of MIA often, there really isn't a huge advantage on any one airline. PBI pretty much will get you to the Northeast and some midwest/Texas hubs.
The only nonstops to the west coast from FLL are Delta (LAX) and Virgin (LAX/ SFO). youre going to have to stop at least once otherwise. (There is a FLL-PHX on US.)
Delta has the most destinations of the legacies, but if youre connecting anyway, probably not a huge difference.
The caveat is your "middle of the country" trips. Depends where they are. If they are DTW, MSP, or CVG, you're better off with Delta. If they are ORD, AA or UA, and DEN or CLE, UA.
If they're smaller markets, look to see who has the best onestop connections (IND, CMH for example are also Delta).