The definitions are settled airframe practice, not marketing
Wet wash and dry wash are not two brands of the same service. In standard airframe and powerplant doctrine a wet wash applies a cleaner by spray or mop and rinses it off with running water; it removes oil, grease, carbon deposits and most soils, and it does not remove corrosion or oxide films. A dry wash removes airport film, dust and small accumulations of dirt where the use of liquids is neither desirable nor practical, and it is expressly unsuitable for heavy carbon, grease or oil, especially in the engine exhaust areas. That second definition is a limit written into the method itself, not a limit on the crew performing it.
The variable is soil load, not quality
Below a certain level of contamination a dry wash lifts the film into the media and carries it off the surface. Above it, the film stops releasing, the media saturates, and every further pass is abrasive. You are then polishing the paint with the dirt you failed to remove. That mechanism sits behind almost every set of wash-induced swirls we are asked to correct, and it says nothing about dry washing being a lesser method. It is dry washing being asked to do a wet wash's job.
Where the aircraft stands decides this as often as the aircraft does
Spent wash water is not clean water. It carries what comes off an airframe: suspended solids, fuel, hydraulic fluid, oil and metals. Fields handle that differently, so the first question before any wet wash is where the runoff goes on that particular stand, and the FBO or the airport authority answers it. Where there is no containment there is no wet wash on that ramp, whatever the soil load says. That is the case dry washing exists for, and a dry wash executed properly at a soil load that suits it is the better outcome than a wet wash improvised with no plan for the water.
What choosing wrong actually costs
Two different bills, neither of them a price list. A dry wash on a heavily soiled airframe mars the finish, and marring is removed by correction, which abrades the clearcoat. An aerospace clearcoat builds to roughly two to two and a half mils dry film thickness, it is finite, and the only way to put it back is a strip and repaint measured in five to seven weeks on the ground. You have spent a consumable film to fix a wash that should have been booked as a wet one. In the other direction, a wet wash with no plan for the runoff is an environmental problem attached to your tail number, and the belly soils a dry wash was never going to reach, hydraulic fluid, fuel, exhaust soot and grease, sit there trapping corrosive material against structure.
What changes the answer
Exposure, season and where the aircraft sleeps. A winter of de-icing fluid, a long spell parked outside, or heavy staining aft of the nacelles all move the answer to wet. A hangared aircraft in frequent service, carrying nothing heavier than airport film between trips, stays on dry. Filiform corrosion under paint is inhibited by keeping the aircraft below seventy per cent relative humidity and by washing off acidic airborne contaminants; Georgia is not a dry climate, so the washing half of that sentence carries more weight here than it would in Arizona. Frequency is not ours to assert either: the FAA's corrosion control guidance leaves inspection intervals and treatment with the operator, the manufacturer's recommendations or your own program.
What separates this from a fast ramp wash
Chemistry, media discipline and containment. Cleaners qualified to AMS 1526 and Boeing D6-17487 have been through sandwich corrosion testing, immersion corrosion, acrylic crazing, paint softening, and a 150-hour hydrogen embrittlement test to ASTM F519 on notched cadmium-plated specimens held at 45 per cent load. A product that was never submitted cannot have passed. Then the labour: media changed far more often than looks necessary, one panel at a time, and water captured rather than allowed to run. A fast wash is one person, one product and a couple of hours on the stand. It is a different product from this, not a cheaper version of it, and what separates them shows up in the paint rather than on the day.