This is really a factor of understanding Ohms Law and the power curve of the alternator.

If all parts of two identical turbines were designed properly (12v and 24v), there should be minimal difference between the two at what RPM they stalled. The stalling usually takes place when you attempt to load the alternator before the prop gets up to an acceptable RPM. An easy way to avoid this is to make sure you wind the stator for cut in at a reasonable RPM. If you shoot too low, you will stall the prop and prevent the machine from working optimally in higher wind speeds. Sometimes this is acceptable because in most battery charging systems, the bank will be charged early on and some of the power from the higher winds can be wasted by allowing the machine to furl,...in other words, it makes for a slow, peaceful machine. This is not optimal for grid tie systems however. A compromise must be made, and in most cases, MPPT will help out big time by widening your operating "window".

With regard to the voltage difference and stall, this has to do with the resistance of the electrical circuit running up and down your tower. The circuit is comprised of the stator, the rectifier, and the feed line coming down the tower. The feed line and the stator must be thought of as one piece. If the resistance calculations done when building a stator, took the feed line resistance into account, I think it would be a little better understood. A 12v machine will have 2x the current as a 24v machine. This means that a larger feed line will need to be used to avoid line loss. If you use too large a feed line, you run the risk of stalling. Too small a feed line, and you run the risk of over speeding in high winds. It is important to have the right amount of "coupling" to have a workable system, and still attain stator longevity. I believe the latest trend is to use larger magnets than normal. This will allow you to have a tight coupled rotor/stator and tends to make the stator less prone to burn out from high winds because you can use less copper and keep your resistance lower (furling is still very important). The resistance is then added to the feed line to make up the difference.