Wind generator power is directly related to the "swept area" of the blades, or to blade diameter. While efficiency does play a part, nothing beats swept area to increase the output. The "large" ones are, for example, Windbugger, Four Winds, KISS. The smaller ones are Aerogen, etc. Practical sailor tried to do a study at Miami boat show this last winter, where all the wind generators were on display and connected to Link 10''s, but there was insufficient wind. However, Practical Sailor is now doing a survey, which should be out soon, and should shed some light.
The biggest issues are reliability, power output, customer service, and especially noise. Only the Air-X was real noisy in Miami. Output of all of them in less than 10 kts is dismal. Of the small units, I liked the Aerogen 6. Of the large units, the KISS looked good and the 4 Winds.
I have a Windbugger, which came with the boat, on a post aft. It is a large unit, so power output is reasonable. However, customer service is dismal--I had it in for repair and was not happy. I would think hard about buying another one. KISS gets generally good reviews and they now have a bearing that allows full 360 degree rotation, which they didn''t have before.