I have a membership with all the major ones since why not since they are free.
Status:
National: Executive Elite
Avis: preferred plus
All others basic membership.
I do 98% national, 1% Avis and 1% other for my rental needs and I can compare Avis to national and I prefer nationals over Avis IMHO. Generally I find the cars better with National and the rates are most of the time better for the same class car. Avis promotes you can get a free day with 700 points but where I regularly rent from its not possible. Used like 2100 points for one day.