Avis Presidents Club is supposed to get a 2 car class upgrade. In practice the very best cars will go to those paying for them, then Chairman members. But the Presidents Club members I know will get pretty much whatever they want that's left after those two groups get thieir vehicles. That usually means that the better Cadillacs are gone (sometimes they're gone for CHM members, too).

A Towncar is a good bet off a full size, but with Presidents Club status you should pretty well be able to walk up to the preferred counter and ask for anything else that's available in the lot.

You don't get the valet service given to CHM members, but many stations will ensure that a Presidents Club member's pre-assigned vehicle is closest to the entrance/exist as appropriate.

And though rarely used, guaranteed availability is a huge benefit.