I'm 5 star with Hertz and only preferred with Avis. Even though I technically don't qualify for upgrades with Avis yet, I get them around 30+% of the time. In the past year or so, all from a reserved intermediate: 2x SUV, Buick Lucerne CXL, Impala LTZ with leather, and a couple of other full size. Some of these cars were pushing 30k miles, but as long as the don't smell like smoking cars and have no obvious mechanical problems, I don't have an issue with that. I typically keep my own cars until they have 150-200+k miles, so even 30k is pretty young in my book.

As a 5 star, a lot of the Hertz "upgrades" are "in-class", so when I reserve an intermediate (C class), I end up with a "YC", or "premium" intermediate, which generally means leather or sunroof, but really not a better car. Hertz also seems to have more category slippage, such as counting a Toyota Corolla as an intermediate. I got a YC Corolla once as an "upgrade" which didn't have any obvious premium features, unless you include power windows. While I have occassionally gotten a low mileage car from Hertz, overall the mileage of the cars I get from the two companies is similar.

Speed of rental is about the same, in both cases usually pretty quick.

So it is pretty close, but my luck has been a little better with Avis so far. I have enough rentals now to qualify for preferred select, so hopefully once that kicks in, it will be slightly better still.