All things being equal the 12v machine would be the least likely to stall as the line resistance has greater effect and so does the diode drop.

I suspect that many people pick up winding details for higher voltage machines and try to use them at 12v. There is a mistaken belief that using more turns gives better results and it doesn't and can really spoil the performance.

As you say, stall is related to blade speed and angle and the system voltage doesn't come into this directly.

The best you can hope for is to choose a cut in speed where the blades are rotating above the ideal speed but still near enough to give some results in the lightest wind.

The tsr falls very rapidly with load and you will soon come on to the peak of the power curve. As you load more you fall below optimum and if you get things right you should just be approaching stall at the time you need to furl to protect the alternator.

If you cut in too early you start on or below the peak of the blades curve and you run into stall within the working wind speed range. Once you hit stall badly the power remains virtually constant with increase speed.