I know I'm a bit late in responding, and you probably have your plans set already, but:

Absolutely recommend a car in Curacao. The best beaches (in our opinion) are on the northwest side of the island and not to be missed.

Driving in Curacao was easier than on SXM; roads are better maintained (yes, there are some potholes to dodge), a lot less motorcycles, and they are just easier to navigate, especially in the countryside. We rented from D&D car rental, one of the locals by the name of Daniel. You can google it. He came highly recommended on TripAdvisor. He'll pick you up at the airport and his office is a block away. He's very simple to deal with. Always responsive, deals with a cash system, extremely friendly. His cars come supplied (for $5) with a wonderful GPS, which he pre-loads with all the essential beaches and tourist places you'd ever want to visit. You just go through a list and touch where you want to go. It was extremely helpful, as noted earlier, some of the roads are not marked.

Enjoy your trip! We loved Curacao.