buy the full insurance. If you wreck, they will put you in jail until you can prove that you are financially able to pay all damages. Also, risk of damage to car with normal driving-pot holes, stars in the window. If you mess up the car, you are at the rent a car co.'s mercy.

I never buy in the US but always in Mexico

Herts, Avis etc.. large co.'s are very expensive compared to regional. You may want to make reservations (that you can no show without penalty) and when you get to the airport, bargain with everyone. I have had a confirmed reservation for 35$ a day and was able to rent the same car for 13$.