The only reason it's double is convenience. It's so easy to just walk to the next gate and hop on a flight. The taxi ride to muni is about $10 and takes maybe 15 minutes through town. Not scenic. But it IS cheaper. I've done it both ways, and if you're budgeting, take the taxi. With the money you save you can buy a round of beers at BC's...
Can't help you with the Miami airport. I know they rent lockers, but I doubt they're big enough for a lot of luggage.
Debbie