This is funny as I had once reserved a Premium SUV (Suburban) at PHX because I had 7 passengers and needed the extra cargo room in back. Arrived at the garage and they had the valet pull up a Tahoe. Stated that I reserved and needed a Suburban or similar. They replied there's no difference and they didn't have anything bigger than the Tahoe. I distinctly remember the rate being more for the Premium than the Full Size. Wrote in to National asking for the rate difference refunded and they replied "nope, no difference".