In terms of earning rental credits, I don't think you can beat National. Also, depending on location, National may have lower mileage cars.

On the other hand, I find that Avis rentals usually have lower taxes and daily rates.

It also depends on the ongoing 3rd party promotions. The majority of my rentals over the past few months have been with Avis rentals due to the AAdvantage/Carlson points. Looks like I'll be switching back to National to double dip on 1-2 Free days and United MP points.