They're really two different markets. Do you want solely the cheapest price (Avis), or the best service and cars, with the least hassle (National)? I would note that 'upgrades' are a poor comparison - the ones I've gotten with Avis have been to undesirable pickup trucks and giant SUV's when I wanted a sedan for city driving, whereas with National, you just go to the aisle and pick what you want, no 'upgrade' needed.