There are a few factors for me:

1. Quality of car - before PC gave you "choose your own car" I mostly used to rent premium/luxury in the US. Now I can rent midsize for typically $50 a week less.

2. Quality of service - no point in saving $10 on a car, just to have to take an extra shuttle and wait in line for an hour and then do the same on the return. Similarly, no point in getting nickel and dimed on the return with extra insurances, rejecting the ones I have, etc. I'd pay $0 for this element from Hertz over Avis, Enterprise, etc., but I'd pay $50 a rental before taking a punt on an unkown provider.

3. Ease of retaining status - with my current rental patterns, PC is attainable but not a sure thing. No point in saving $30 only to spend $30 on a rental I otherwise wouldn't have wanted/needed to make up the 20 rentals per year.

So: if an Avis premium is even $1 less than a Hertz C, and I have enough rentals that year - Avis is probably getting my business. On the other hand, a no-name Corolla for $125 less when I'm short on rentals is likely to see me sticking with Hertz.