I would go with a taxi or this private shuttle service a previous poster mentioned (
www.costaricawelcome.com). They charge $99 from San Jose to Jaco. It's probably the same from the airport to Jaco. This rate is for up to 6 people.
Interbus is $29/person, so it makes sense to take a taxi or private shuttle if you have 3 people.
In Jaco, you don't need a car, you can walk from one end of Jaco to the other. You don't need a car. Taxi everywhere, it's dirt cheap. You don't want to worry about crackheads breaking into your rent-a-car. (You know that you pay for everything, up to the deductible amount, even with insurance.)