Mike,
Whilst, like yourself, I use a motorised head but when not specificaly doing an asthma treatment I find it more convenient to attach it to a Victor D9 tub vac rather than have 50+ foot of heavy hose to drag arround.
The advantge of the motorised head over an upright vacuum is that, provided you have sufficient vacuum power to sustain airflow, the head seals to the carpet, drawing air through rather than hovering above it drawing air across it.
For large areas I use a Clarke 22" pile lifter.
John.