Since we have gold status at Hertz and preferred status at Avis those are our main two rental car providers for most of our trips. This Fall it seems Hertz edged out Avis due to better Mileage bonuses by Hertz with United. TIP: Always check your airline websites for rental car promotions - there's always a special offer or way to earn miles on your rental!!
Our previous trip to San Diego was lucrative since Avis offered 5000 Delta Skymiles for 2 day rentals.
A word of caution concerning Enterprise - apparently they are not taking the safety of their customers seriously since they just ordered 66000 Chevy Impalas without the standard side-curtain airbag. They save $175 per vehicle but it's detrimental to anyone riding in one of those cars. Enterprise won't get my business anytime soon because of this.....