Coverage is really good down here from T-Mobile. I'm on MetroPCS, which uses the T-Mobile networks, I bought the Note 4 from T-Mobile and used the MetroPCS sim card.
Coverage is great in Miami, At least from Homestead to Palm Beach Gardens, which is where I'll try it out more so. Band 12 has been supported since the beginning of the year, so even deep in the buildings in downtown, you will have good support. Wifi calling has been available for a while, I began using it back with the Galaxy S4.
Download speeds are fast. Usually I'm on Band 4 and download speeds in the afternoon can vary between 30Mbps-50Mbps. Overnight, download speeds are 60Mbps to a peak of 80Mbps. Still on 15MHz, once they increase it to 20MHz, download speeds will improve, not that I even need the high speeds I currently get.
I just chose MetroPCS over T-Mobile because it uses the same network and it's $55 flat for unlimited talk and data. Only pain is getting T-mobile to sell you the phones upfront without the plans, but once I jump through that mess, I don't even have to unlock the T-Mobile phone.
But you will have a great T-Mobile service down here.