Well, IMHO in terms of value for money and for comfort incl. interior and trunk space, and provided 'brand prestige' is not the issue, my vote goes for the Hyundai Azera Limited, same rental class H as the Avalon but a far superior car ... including XM radio and 6-CD changer ...

For another opinion try:
http://www.thetruthaboutcars.com/?p=531

The odds of getting upgraded to an Audi A6 for the same H rate are one in a thousand (!)

And oh yes, I have been driving one of these from Hertz for some time now ...