I have categorized the different carpet cleaning processes going from weakest to strongest. I do all but recommend truck mounted steam or dual process carpet cleaning.  They clean the best and are the only one recommended by carpet manufacturers (see below for references).  Advantages/Disadvantages are in my opinion.

Chemical Bonnet:
Advantages: Fast drying time (about 2 hours).

Disadvantages: Works by encapsulating soil, blending in and absorption by bonnet. Encapsulated soil in the carpet should be vacuumed to remove residue along with soil the next day by customer. Since little water is used Carpet Chemical needs to be stronger then steam yet leave no residue an unlikely combination. Process has limited carpet cleaning power and strong carpet pre sprays cannot be rinsed out. For this reason a customer must accept that it is a light carpet cleaning. Blended soil darkens areas.

Shampooing:
Advantages: Dries in about 4 hours. A brush is used instead of a bonnet so soil in carpet is worked to a greater degree and more is loosened then Chemical Bonnet.

Disadvantages: Works by encapsulating soil only. Since more chemical is used (then Chemical Bonnet ) more must be vacuumed out of the carpet and will therefore have more carpet residue.

Truck Mounted Steam:
Advantages: Carpet cleaning rinses and removes soil from carpet by strong vacuum and heated solution. Can be done as rinse only for soft bright carpet. Inks and colors can be removed with little blending problem. Spotting carpet chemicals are rinsed and therefore can be stronger, so red spots through urine spots can be attacked more effectively. Since water is the primary component may or may not leave carpet residue depending on chemical used. I use Proscoice Powder or Last Step, depending on soil so no residue is left.

Disadvantages: Long drying time (12 hours). If to much carpet chemical is used in wand may have same requirement for post vacuuming as shampoo process does.

A variation is using an RX-20 Rotary carpet wand this uses trucks vacuum strength to spin 3-4 internal mini wands. I have not seen the RX-20 do a better carpet job as you can not edge rub like you can with a wand, or spray continuously on a single spot. The RX-20 doesn't get corners or go under objects as well. It isn't a degreaser or a brush like a power scrubber it does not take the place of pretreatment of spots. in fact it is marketed to cleaners as a way to save fatigue. Please don't confuse this cleaning process with my dual process cleaning below.

Dual Process:
Truck Mounted Steam with Rotary Scrubber

Advantages: Carpet is gone over twice. First carpet soil is degreased with rotary machine (which has a brush and chemical dispenser).  The Chemical is actually an emulsifier called pro powder so leaves no residue. Then Carpet is steamed out with truck mounted using Pros choice Last step, to leave carpet at its cleanest possible level soft and bright.  (see steam cleaning above for link about our chemical choice).

Disadvantage: Long drying time (12 hours).