I have rented from Hertz and National many times. I have no experience with Enterprise and only one rental experience with Avis.
I am a President's Circle member at Hertz. This means I rent from them
>40x per year. For National, they have our corporate account so I rent from them whenever I'm traveling via car on business.
It is true that Hertz and National tend to be more expensive than Dollar and Budget. My best guess, having not rented fronm Avis or Enterprise at an airport, is that rentals from an airport location don't have a whole lot of difference. Airport locations are where most rental car companies keep the biggest fleets (compared to in-city locations) so you normally have no issues with availability and the like.
That said, the Gold/Emerald Club service from both Hertz and National (only at airports) are very nice. You skip the counter entirely. You just take the shuttle bus to the location, and you pick out whatever car you want from the Emerald lot (National) or you've got a car waiting for you (Hertz). No lines, no nothing. Just get in the car, show em your driver's license and go! Very convenient.
For in-city rentals, it varies from place to place. Here, the National/Alamo (they're combined) site often has long lines while Hertz does not. It's weird. They're across the street from each other and Hertz might be $1/day more. I have waited 30+ minutes in line at National before but never that long at Hertz. For $1/day more, it's not a big deal.
I know you're looking for a SUV but I like how Hertz has some specialized cars you can request. Hertz has a Prestige collection (at select locations) where you can rent Jaguars, Volvos and other nicer cars. Even the "regular" collection has some nice cars. The Hertz location in Philly has Toyota Solara convertibles, Infiniti G35s, Toyota Celicas, etc.
Last, Hertz is owned by Ford, which means they have the 2005 Mustang! I have rented the 05 Mustang several times already and it's a welcome change from your boring everyday rental car.
I got the new Thunderbird once but that's a rarity.