Duct Cleaning - what size vacuum should be used

As a qualified duct cleaning contractor I have discovered that most homeowners feel that they are very uninformed when it comes to understanding what size vacuum should be used to clean their air duct system. We are continuously asked weather we use a portable vacuum or a truck mounted vacuum and 90% of the time the customer really has no idea why they are even asking this question except for the fact that they have been told that either a portable is better or that a truck mount is better. Of course most Duct Cleaning companies that you call will inevitably tell you that their vacuum is better than the other guys