It really should be about what you want to do when you are not at the beach. The weather will be fine. It will not be a ghost town in PCB though it will definitely be less crowded. The "offseason" keeps getting longer and longer.
PCB has more things to do when you are away from the beach. The golfkarts, amusement rides, old time arcades, Gulf World. PCB has Pier Park which does go down to the pier as the name states. Destin has Destin Commons but that is like pretty much any other outdoor mall.
30A is for the folks that want to feel and experience more "upscale" environment and food. Also, unless you are insane rich you dont stay on the water, its mostly houses a short walk from the beach. If you want to get up seeing the beach Id say PCB or Destin. I dont think there is too much of a difference between PCB and Destin and I would go with whichever you can find the best deal. There are a few nice condos including some that have near indoor water park quality like Splash from what I have heard.