That depends on who you ask and what they do. If you ask someone who cleans by extraction most likely they will tell you the reason hot water extraction is best. If they clean using a dry methods they will most likely tell you the reasons dry foam is better.
What do we think?
We have done both methods in the past and have a few reasons why we continue offering a dry foam method over an extraction method. Even though it is called dry it does use water. A better term for dry foam is low moisture. Extraction methods use much more water than dry methods, no matter how good the technician is. They still end up using 3 to 4 times the amount of water that our dry process uses. The worst part about this is that the over use of water, as most people here in Florida know, is a waste we cannot afford. With our limited water supply, this is a luxury we really should protect.
After cleaning, the water wasted during extraction is sucked into a holding waste tank. Any good extraction carpet cleaner will dispose of this waste at the appropriate areas (car washes, etc) where waste water can be disposed of. Unfortunately though, you don't know if they are just driving down the street and dumping it into the sewer. This is a big problem (and also very illegal).
With our low moisture process, you never have to worry about that at all.
The second big reason we choose to use a dry method of carpet cleaning is also very much about environmental issues. A truck mounted extraction cleaner's machine is ran on gas. This adds CO2 emissions into our air, which adds to the air pollution we all have to live with everyday. Our dry foam machines run on electricity and do not have any CO2 emissions at all.
Beyond external environmental concerns we have about extraction cleaning, there are also internal household concerns. Since the extraction methods use far more water to clean your carpets, it means they take longer to dry. The longer water sits in your house, the greater the chance for mold and mildew to grow. Our low moisture dry foam process dries in an hour, which greatly reducing the chance for toxic mold to grow in your carpet.
Don't get the wrong idea. We do think extraction methods have very good uses. When water pipes burst in your home, you need someone to extract that water fast. During situations such as those, extraction methods are very effective and necessary.
If you research both methods, everyone thinks they have the answer. Everyone has a bias. Whether it is personal preference or just the business the person is in.
Based on our environmental concerns, we sleep better at night knowing how we clean carpets.