Both my wife and I are National EE and Hertz 5 Star before this promotion. We rent mainly at LGA and MSY.
Both National and Hertz absolutely suck at LGA. If you get to National after 10 AM the Executived Aisle will typically have mini-vans and more empty spaces than cars. Hertz will have the low end models of each category assigned to you and unless you want to spend extra time fighting for a different car, you're stuck with crap. They'll be happy to "Upgrade you" for ridiculous $.
National at MSY is different, maybe because we've gotten to know many of the staff. There's usually a good car selection and if we need something special like a minivan to haul stuff or people, they've always accommodated us. Hertz is better than they are at LGA but still not great.
We've evolved to shopping on price. Hertz prepay, when available and if you can use it, can be a big saving, usually about a 33% discount.
Booking through DL is expensive, we can always do better with the AAA rate.