The next rating criteria for air compressors is CFM.
CFM stands for "cubic feet per minute". It is a measurement of volume. Basically it is how much air is being moved. Air tools require a certain amount of air to run on. PSI is just part of equation. Don't be confused by different CFM ratings at different pressures. Every manufacturer is trying to make their product look better by giving higher CFM ratings at different pressures. The only real concern is how much CFM you will get at 90 PSI. Remember 90 PSI is what most air tools require to operate. To find out what your air tool needs to run, just look on the box for the manufacturers specs. Generally, air tools require 4 - 6 CFM. A good rule of thumb on air compressors is you should get 3 -4 CFM per real HP at 90 PSI.