though Id like to profess expert knowledge on this subject, I must defer to the cust serv number on this one. Essentially, Presidents club gives you....
guaranteed car at any location regardless of sell out status at the location you are at. If the location is 100% sold out, call the 800 number and they will force override a car, or simply walk up to that location. Does this work? yes, likelyhood of getting a chevy aveo or SMALLER, just as good.
When reservations are booked, a based upon availability two class upgrade is applied.
Chairmens club guarantees you the best car on the lot (minus specific specialties like tbird and hummer) when you book a mid size or greater. your own special number/concierge service where the agent will help to arrange car deliveries, additional drivers at no cost, greeted at the terminal by a manager or the highest ranking non management personnel on lot, vallet service where they jump in the car you drove and drop you off curbside instead of skytrains or busses.
both are invitation only and both are dependent upon your companies relationship with avis. for every x dollars of business, you get x number of pres club memberships (and believe it or not, amex has to steer x dollars in business to give this service to its centurion members). The same applies for chairmens memberships but the number is less (for every 10 pres club memberships, you get 1 chairman membership).
disclaimer, I do not know the exact ratios, or dollar amounts, hence I say im not an expert. Companies like Microsoft or Boieng are likely to have 10 Chairmens club memberships, 150-200 presidents club memberships. That should give you an idea on the size it would take.
incidentally, this is one reason why companies want you to use their AWD for personal trips. it puts a dollar total in the companies ledger towards negotiating discounts as well as towards PC and CHM memberships.,