If you prefer mazda 6 just call and ask to put mazda 6 in your profile.
This in the past greatly increased the chances to get my preferred car.
Unfortunately mazda 6 is not as common at hertz as it was 2-3 years ago so it's still quite a YMMV.

While National is a good option and you get to pick your car on the lot, more often than not all you are gonna be picking is your favorite color among a dozen of so of pontiac G6s