further research says the MEGA
drops you off at the rental car center which has free transport to the air terminals
but I suggest you verify . . .
I know for a fact that
Red Coach drops off at MIA Terminal J, which is the last terminal on a U-shaped airport. However, there are moving walkways on the third level that will take you anywhere in the airport.
I have several friends who use Red Coach regularly -- two for WDW and another to see her daughter at Florida State. They rave about the accommodations and ride.
Mega Bus says it drops off at the "Intermodal Center" at MIA, which is the hub that connects the airport with the rental car industry, Tri-Rail (rail service to Ft. Lauderdale and West Palm Beach), bus transport, and MetroRail (elevated rail service to downtown, south to Kendall, and northwest to Hialeah.
Megabus' website notwithstanding, their dropoff location is NOT near downtown, and there is NO connection to South Beach.
The Intermodal Center is across a main thoroughfare from MIA (about a mile away), but is connected by autonomous elevated rail car to the terminal.
But MIA is OP's goal, so the Intermodal Center would
work.
However,
debarking at Terminal J from Red Coach, even if your flight was on American Airlines on the other side of the airport, would be
easier...especially if you have significant luggage.
I don't know anything about Mega Bus.