The following is a summary from Prestone Corp.:

-PG IS LESS TOXIC TO MAMMALS

-BOTH RELATIVELY NON-TOXIC TO AQUATIC LIFE

-BOTH READILY DEGRADE TO CO2 AND H2O

-PG IS LESS EFFECTIVE AT LOWERING FREEZING POINT AND RAISING THE 
BOILING POINT OF WATER

-PG IS MORE VISCOUS AT LOW TEMPERATURES

-PG CONCENTRATIONS CANNOT BE DETERMINED WITH HYDROMETER

-PG COOLANT IS A POORER CONVECTIVE HEAT TRANSFER FLUID THAN IS EG 
COOLANT AT SAME CONCENTRATION

-PG COOLEANT PROVIDES COOLER METAL TEMPERATURES AT CYLINDER HEAD HOT 
SPOTS UNDER BOILING CONDITIONS THAN DOES EG COOLANT AT SAME 
CONCENTRATION

-dan