Renting on line is the way to go here, allow yourself time to select the vehicle you want at the price you want. Rentals at the airport are not generally more expensive and may at time be less however there are so many agencies at the airport, when they see you are looking they will be after you in mass, it just creates delays and confusion.
===============
Hertz Sint marten offer the most locations and largest fleet -you can book direct or from any discounter on line, you get the liability coverage included in the rate no matter how you book.