Usually, the off airport locations are cheaper because there will be an added airport fee for cars rented at the airport. However, if you are flying, it may be hard to get to the off airport location to pick up the car.
I've had the best luck by going to the rental car websites. I book a car and then check back a few weeks later. If I find a better deal on the same sized car, i'll cancel the original reservation and rebook. I find the best deals about 3 days before I pick up the car. By doing this on our recent WDW trip, I went from $350 for 9 days down to $215 for the same number of days and same sized car.
If you are a Costco member, you can also get discounts on car rentals through Costco Travel.