I used to only use Dollar; booking premium cars yielded such nice rentals as the Chrysler 300, Dodge Magnum, Dodge Durango, Chrysler Pacifica, Jeep Grand Cherokee, etc. You can usually book a premium car for less than the Midsize with Avis or Hertz. The only problem with Dollar is they have horrible customer service at some locations (JFK, BOS, BDL, PVD) which are airports that i tend to use a lot, so the 50 miles is no good.

Since most give 50 miles per day, and Hertz and Avis give 500 for 5+ days, I'd pick one of them. They are both usually in the terminal, or they're the closest to the terminal, with the most shuttles passing through the airport. If you are at an airport with off-site rental cars, count the number of Avis or Hertz buses compared to say a Dollar or Thrifty bus. You'll see at least three times as many.

Avis Preferred is free, which is nice. Hertz's #1 Gold costs I think $50 per year. You can get the "regular preferred" service with Hertz for free (I think it's called Hertz Gold) but you still have to go to the counter, insead of straight to the car.

If you don't mind GM cars, with the occasional Ford, then go with Avis. If you can spend a bit more, then Hertz should be your choice. They have some great premium cars (Volvos, Jaguar, etc).

For me? I've switched almost all of my business to Avis. Great service, but every now and then I end up with a darn Chevy Classic.