Use a company that will provide a fair quote, based on the service that they will provide. Be weary of companies that advertise one price for a whole house. Think about it, would a painter paint any size house for one low price? Neither can an air duct cleaner PROPERLY clean the WHOLE house, no matter how many systems or air vents are in the home for one low price.
On the other hand there are many more air duct cleaning companies that will properly clean the air system and air ducts and use commercial negative air duct cleaning vacuums with at least 2,000 CFMs of vacuum power and utilize proper agitating tools to move the contaminates within the duct work to be captured with HEPA filters and contained by the air duct cleaning vacuum equipment.