There are lots of misleading vendor claims and some downright lies.
All vendors have to work within the same laws of physics (For you politicians, remember that these laws CANNOT be changed by man, or marketing genius)

There is a defined and easily calculated amount of energy in a square foot section of moving air (i.e. wind)
The amount of energy that a windmill can extract can never be more than the energy in the wind. The actual amount of energy recovered depends on the blades efficiency. There is lots of science available from big windmill research, so any moderately smart builder of windmills for yachts will make his blades close to the maximum possible, which I believe is around 55% efficient.
From this energy there are small losses converting the mechanical energy to electricity.
Thus all windmills will get pretty much the same amount of energy from a square foot of wind.
The bigger the wheel, the more energy can be captured.
I bought KISS several years ago mostly because of its size (5 ft diameter) and also because of its simplicity. The smaller wheels have no chance of beating its output over a range of wind speeds.
Do not be fooled by windmills spinning merrily while the KISS does not turn in low winds. The spinners are not generating any power, or are generating only a trivial amount.

BTW, we have been pleased with the service from John and Libby at Hotwire.