The term 'steam carpet cleaning' would lead you to believe that no chemicals are used, only steam. This is not the case. Steam cleaning requires vast amounts of water from a truck mounted or rented machine. The user goes over an area with a mixture of water and chemicals, and then covers the area again using the machine to suck up the water.
This is the only system that can claim to lift stubborn stains and dirt as well as reach the bottom, or pile, layer of carpeting. If you have high traffic areas or stubborn stains on your carpets this may be the only reason you need to choose steam carpet cleaning versus chemical carpet cleaning as your best option. The drying time is shorter than with other wet methods, because the suction of the machine removes the water and the lack of water in the carpet decreases drying time.
Chemical cleaning is a process that is also referred to as dry cleaning. It should be noted that it is not dry but does use 80% less water than the other method. The upside of this is a return to your area quickly. It is also worth mentioning that mold and mildew are not likely to grow in a quick dry application with this method. The fast drying time is probably the biggest benefit to chemical cleaning. The downside of this method is that it is unable to agitate the bottom layer, and therefore, resistant stains that are deep in the carpet may end up remaining behind. Also, the chemicals used could possibly be left behind in the form of residue and attract more dirt to high traffic areas.
The bottom line is that the steam carpet cleaning versus chemical carpet cleaning is a personal choice that is best decided after identifying your needs. If you need a fast-drying solution and do not have really deep stains, chemical cleaning may be for you. It you have deep stains and can wait a bit for drying, go with steam cleaning instead.