This is what I could get from avis.com and other posts around FT. Hope it helps.

Preferred ~ Entry Level

* You won't have to tell us your rental preferences every time you reserve -- we'll already have them on file.
* When you arrive at Avis, you can skip the lines and the paperwork and go straight to your car at over 1,400 locations worldwide.
* Our courtesy bus will drop you off by one of our newest cars, or you'll find it in a convenient parking space.
* When you enroll in Preferred Service online, you will also enjoy automatic enrollment in My Avis -- the personal service dedicated to enhancing your online Avis experience.

First ~ 12 or more qualifying rentals, or 35 or more qualifying days in a calendar year

* Continued access to Avis Preferred Service – skip the lines and go directly to your vehicle
* Free weekends upon invitation and membership activation
* Free weekend rentals of time and mileage can be earned with every third qualifying rental (each rental must have a minimum of 2 consecutive paid rental days). See reward fulfillment email for full terms and conditions.
* Free one-car class upgrades when available
* Dedicated reservation line and customer care
* Members-only Web site to view rental activity, rewards and special offers

President's Club ~ By invitation or with certain credit cards (Amex Cent, Continental Presidential Plus MasterCard, others?)

* Guaranteed car availability (will set up reservation for you even if sold-out)
* Two car class upgrade (based on availability)
* Car is ready up front and ready to go
* Savings on international rentals
* Members-only reservation line
* Can be member of both Avis First and President's Club, and thus earn free weekend rentals

Chairman's Club ~ Invitation only (plus possibility of an annual fee (eg.: with Amex Plat card - $350 fee, but with Amex Cent card - no fee))

* Guaranteed car availability (no reservation required, can just walk up to a counter and get a car)
* Can ask a Avis rep to meet you at the arrival doors so you can get in and drive off.
* Guaranteed three car class upgrade (?) OR best available car when booking mid-size or larger (excludes specialty vehicles)
* Pay market rate (or lower) for gas, so no need to refill before returning
* Vehicle delivery - Avis will bring you a car within 20 miles of an Avis location
* An Avis rep will drive you back to your departure gate when returning your car
* Dedicated Chairman's Club CSR desk
* Can NOT be member of both Avis First and Chairman's Club, thus can NOT earn free weekend rentals