I have recently stayed at the Westin Colonnade in Coral Gables.
Pros:
Lots of restaurants/shopping within walking distance
Beautiful lobby
Gorgeous pool area
Fairly centrally located to what you want to do

Cons:
Did have some partiers there when we stayed and got a bit loud.
No lounge area for preferred guests
Valet parking only
South Beach is a haul


Not that you will, but don't stay at the Airport Sheraton. It is a dump.

Don't know much about the other brands you mentioned. The Conrad Hilton is in a nice area centrally located with lots to do around there.