When you rent from National, you go to the garage across the street from the terminal and pick the car from the class you rented from. If they have ten mid size cars, and you rented a mid size, then you have ten cars to choose from, your choice. The keys are waiting in the ignition.
Over at Hertz, they give you the parking space number of where your car is and chances are, you'll have to learn to like that car.
Hertz is located about 2 miles north of the airport so they have to shuttle you. On site rental car agencies cost the same, if not less, and much easier to get to. Right below the baggage pick up is the car rental agencies. Can't get easier than that!