Number 1 criteria in selecting a wind turbine

Propellor Length. The bigger the propellors the more power you can generate from the wind. So the number one rule of thumb is get the wind generator with the largest propellor. Ok why don't all wind generator manufacturers put huge propellors on their turbines? The answer is that they can't.
For a given alternator or motor, there is an optimum propellor length, to maximise the power that can be generated. Too short and not enough power is generated. Too long and the RPM starts to stall and slow down as well. The propellors must therefore be tuned to the motor and there is a maximum optimal length after which the output actually declines.
Knowing this we can now deduce that when choosing between wind generators we should look for the longest propellor in our price range. This is like choosing the largest horsepower motor for our money.