HVAC technicians like the quietcool because they like the idea of having an adjustable duct to attach to the grille and having the motorhead fan on the otherside of that duct. This makes the sound of the fan insulated, hence the name QuietCool.
QuetCool suggests that we size the system for between 2 and 3 CFMs per sqft. of your home. So if your home is 1000 sq ft., then you'll want a system that can move between 2000 and 3000 CFMs of air.
With this in mind let's say you go over to the different models and find the Trident Pro 2.5 and Trident Pro 3.3 will move between 2500 to 3000 CFMs of air. The technological genius of QC fans is in the insulated damper that shuts off any access to the attic when the system is turned off. Because the damper has an R-5 insulation rating on it, they do a good job preventing heat from the attic to come into the home when the fan is turned off. In 2011, the QC became the first to incorporate ECM motors with the fan. ECM motors run quietly and at lower amperage than regular PSC motors. PSC motors are the ones that you've seen on traditional whole house fans since the 1960's. They also require a capacitor to run properly, and if that capacitor fails, the fan won't work. So you would have to replace the capacitor in order to make the fan work again. ECM motors are electronically commutating motors.
A good point to add here is that HVAC guys will say that the QuietCool ECM motors in the Stealth Pro line will operate a lot better at lower fan speeds. For example, if you had a 2-story, 2200 sq ft. home and installed a 1.5 Trident Pro in a master bedroom ceiling near the door, and put a 3.3 Trident pro at the top of stairs on the second floor, you'll find that the bigger model will come out cheaper in the long run because it's moving the most amount of air for least amount of power. In other words, we recommend getting the biggest model and setting your three-speed RF switch to the lowest setting for maximum efficiency.