score:4
Parking can get really expensive depending on where you go. There are not much parking spaces available in the areas like the coastal regions of Miami Beach. Also, traffic sometimes get congested as well. If you won't be travelling around much (from Key to Key, and into far neighborhoods) then I would suggest you stick with Taxi/Uber/Lyft.
Miami is a big city, so beware that if you want to see a lot of places in a short time, a rental could be your best bet.