I rented from both Hertz (25 rentals) and Avis (10 rentals).
Hertz has better service at the counter and AVIS seems to have the opposite.
AVIS seems to have better cars than Hertz.
I am not sure how you would guarantee any particular car from AVIS or Hertz. They all seem to be based on availability.
In the end, I would go with AVIS.