Like others have said, Mousesavers all the way.
As for which one is going to be cheapest, it really all depends on which company, what location, what class of car, what dates, and what promo codes you use. It may take some time, but try different codes and see what you get.
Also, remember since you can make a reservation for a car without pre-paying or giving any credit card guarantee (unless you choose some sort of pre-pay option) you can keep checking rates and promo codes up until you actually pick up the car. Definitely check back from time to time and see if any new codes apply. And many times, car companies will lower rates as the dates approach if it looks like they are going to have a lot of cars in inventory.
On most vacations, I end up booking and cancelling car reservations several times with several companies before the trip starts. Last trip I started the search with a total of about $250 dollars for the week, which was a pretty good rate I found with a coupon code. By the time I got there, I was able to find a code with a reduced rate for $120 for the week. It pays to shop around on car rentals!