I rent from both Avis and Hertz regularly. I find them much about the same, although Hertz tends to be slightly cheaper but this seems to depend on location in my experience.

Take a look at this FT forum Car Rental Programs/Partners. There are numerous tips regarding each programme. There are also some useful codes that you can use (and discussions about the morals of using such codes) within those forums, which will provide a discount.

Avis

Hertz

You can often shave 20% and more off the published rate (although miles will not post for some codes).

With Hertz, I signed up with my Amex Plat and received #1 Gold membership and receive (upto) 20% off with the Amex code. I've just made President's Circle but I've not rented at that level yet, so can't comment on upgrades/service.

Palmer