There are many rental companies, national and local, to choose from as long as you have a major credit card and are over 25 years old.
Rental rates range wildly, depending on company and time of year.
Booking via web site can net you extra incentives. Also research fly-drive deals and rentals through your travel agent. Remember that the prices companies quote you do not include state sales tax or either collision damage or liability insurance. If your home policy or credit card doesn't cover you, bite your lip and stump up the extra - even though it may almost double the rental bill. If you want to rent a car for a day or two in the middle of your stay, you don't have to make the inconvenient and expensive trip to MIA - there are local rental companies that will, for no extra cost, either deliver and pick up your rental car at your hotel or pick up and deliver you to their office.