We saw relatives wait to get on the
Avis shuttle last trip and heard the rental office was crowded.
This past December, we rented with national. The office is a little further away, but was no hassle.
Although you have to take a cab to their rental site, National reimburses you the cab fare, so it is more convenience at no cost.
Just tell the baggage handler that you are renting from National and he will take you to a tented area in the parking lot where he will ask the CM to call for a yellow cab. Our cab arrived in minutes, while many people were still waiting for the next Avis shuttle. We had already seen one fill to overcapacity. The National office was quiet and orderly - only one person ahead of us. The cost was well below what Avis was charging.
Although it may seem more complex, it was so easy, quiet and comfortable, we would rent from National again. Also, no extra charge to drop off at airport.