I have memberships in all the car programs but the best for upgrades and service has been Hertz. I am at 5* and have gotten an upgrade about 90% time at airports with the last being a double class upgrade. Sometimes the car is running to warm it up. Hertz does seem to have higher rates but I pay them when not too divergent.

At Avis I have never gotten an upgrade even though it is one of the perks. Budget is OK and you can collect rental points and their fastbreak works pretty well but I get more upgrades at Hertz and a little more service it seems. I do like the National Emerald Isle service but again Hertz seems better.