I have chased the tail on this a bit. It appears that the windmill is the tried and tested product for longevity. However, the solar products are vastly improved and are replacing windmills as the windmills wear out. So one must surmise that the folks that have mills believe the solar product to be at least less cumbersome as relates to maintenance. It is pretty obvious that a walk over to the array of panels is a less traumatic experience than climbing a 40 ft tower and standing on a 4 ft square platform in the wind, with a free swinging mill head that must be oiled, etc.
In my case, without either system and starting out from scratch, the solar is a lot less of a problem to install, especially considering the 40 ft wide spread and anchored (hurricanes) tower for the mill. Plus, at 20 years, I figure someone else will be replacing the panels that wear out.