I'm a PC with Avis and EE with National.

Generally speaking, I prefer the National Emerald Aisle model more than the Avis assigned car model. Sometimes at Avis I'll be upgraded to a nicer car, or they say "Thanks for being PC what car do you want" but those times are few and far between. National, on the other hand, I reserve midsize and almost NEVER drive one -- today my midsize rental got me into an Audi Q5. I've seen lots more used cars in the Avis fleet of late, whereas National seems to be still rocking mostly new cars. So, price being equal, I choose National when I can.