I generally recommend you dont rent cars when in Miami Beach. The reasons being:
#1 The hotels charge you upwards of $20 per day for parking. Its all Valet Parking too so you get whacked for tips coming and going if you actually use the car.
#2 Parking is virtually impossible to find anywhere. This includes all bars restuarants and local shops. You either pay to park at the club/restaurant or you end up walking six blocks or more to get back to your car.
#3 At most times traffic on Collins ave moves at a snails pace. The same applies to Ocean Blvd.
#4 Its EXTREMELY easy to get a cab 24 hours a day in South Beach
#5 Almost everything worth doing on South Beach is within walking distance. If what you want to do isnt within comfortable walking range (or you are gravitationally challenged like me) you can go back to option #4