Should I book by the hour or point to point in Key West?

Point to point if you know both ends and there is nothing in between. Airport to hotel, dinner and home. It is the simpler booking and usually the cheaper one.

By the hour when the day has more than two stops, when you do not know yet how long something will take, or when you want the car to wait rather than be summoned again. Showings, meetings, a shopping afternoon across Old Town, Duval Street, the waterfront, an evening that might end at eleven or might end at two.

The rule of thumb: if you would find yourself booking a third car that day, the hour was cheaper from the start. And if you are unsure when you book, say so. A concierge would rather set it correctly than have you rebook at seven in the evening.