One of the more challenging tasks I have done consistently in my 23 years of carpet cleaning has been to explain to potential customers in a short phone call why they should choose truck mounted carpet cleaning (the large unit that stays in the truck) over portable carpet cleaning (the small unit that is brought into the house).
The short answer is power. The truck mounted unit has a lot more of it. A portable machine has about a 373 watt motor, comparable to what is in your washing machine. A good truck mounted unit has at least a 750cc gasoline engine, comparable to the engine on a Ninja motorcycle, Impressive you say, but what are you going to do with all of this power? The answer is: Heat, water pressure, suction.
The heat that you get from a portable machine comes from whatever is available at your tap and decreases quickly, diminishing one of the most important elements of carpet cleaning. Good detergent gets very average results with lukewarm water. A truck mounted machine in many cases transfers the heat of the engine exhaust to the cleaning water, giving you piping hot cleaning water that makes the detergent very effective.
The water pressure of the portable electric-driven unit is between 90 and 250 psi depending on the quality of the unit. Truck mounted units use about 500 psi. This is good because the higher pressure is able to physically agitate the dirt and remove it from the fiber. More experienced carpet cleaners put the detergent on the carpet ahead of time, then use their super-heated cleaning water to rinse the carpet fibers.
Last but not least is suction: the Holy Grail of carpet cleaning. A decent suction allows you to put a small amount of water down and draw
it out, but a great amount of suction allows you to circulate about 5 gallons of water per room through the carpet, rinsing out large amounts of dirt and pollution. Great amounts of suction means you will have very little of that water left behind.
In the portable machine this suction is provided by a fan that air can simply move around when demand is put on it.
In a truck mounted machine suction is created by "positive displacement". I know it sounds technical (and be sure never to get a race car enthusiast or carpet cleaner started on this subject), but in short it is an air pump that requires a great amount of power but provides insane amounts of suction, giving the air only one way to go.
The cleaning from a truck mounted machine is going to cost a little more for the consumer because the purchase price for a good unit starts at about $13,000, but I hope the information provided helps you to see why it is worth it.