I'll say airflow, but it has to be qualified.
When the machine is down on the carpet, properly adjusted and in cleaning mode, the airflow through the machine is important as it removes the dirt which the brush roll beats and sweeps out. When the machine is not on the carpet, airflow is unimportant. That's why I consider Kirby's 'suction tester' plate that covers the bottom, as well as the Baird meter, to be 'tricks'.
That's one reason I have a Vacuflo central vacuum - nothing to stop that all-important airflow.
