Erm...you could try this:
-select the Miami safe house and go to it.
-exit the Miami safe house and exit the game.
This should now allow you to spawn at the Miami safe house rather than the L.A. one.
I believe that, depending upon what you were doing on the map last, you'll spawn at the closest available safe house, so if you are doing events on the East Coast or South, you'll spawn at the Miami safe house.