National, Alamo, and Enterprise are all owned by Enterprise Holdings. I'm not sure how independant they are of each other. Budget & Avis are both owned by the same company and at many locations they share cars.
Not that any of this really matters.
In your case, I'd look at the fine print on the rental agreements, including insurance coverage. I usually rent from Avis through my insurance company due to much higher insurance coverage which is included in the rental.
I like National because of their Emerald Aisle, (book a mid-size car and if you are a member you can pick your car from the emerald aisle and they often have SUV's and minivans in the Emerald Aisle). I also like that after you arrive you can go straight to the garage pick your car and drive out. You stop at the gate and they check all your info and in 5 minutes you are on your way.
Alamo has the kiosk's and you can do online chech-in to speed things up with them.
If the prices were the same, I would take National (as long as you have the Emerald Club membership)
With National and Emerald Club you head straight to the garage and pick your car. With Alamo, you have sign in at the Kiosk before heading to the garageand picking your car (which more than once have netted us a mini van or SUV when a mid size was booked) . Last time, it wouldn't let me do the kiosk (I was told later it was because I had a coupon code on my reservation - although I think I have had coupon codes before)
I ended up having to wait about 20 minutes.
That was the first time I could not use the kiosk after using it probably our last 4-5 trips.
But nothing's quicker than just walking out to the garage.
I actually got a pretty good rate with National for our upcoming trip and am excited to be using them again.
Anne
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