Of course, the effectiveness of both vane and AP is strongly influenced by the proper balancing of the sails
and even how the boat is trimmed. I have heard anecdotally by people with an interest in power usage that proper sail trim can reduce the energy consumed by a typical AP by 60-75%.. The same sail trim would likely reduce chafe on a wind vane
and wear on its mounts and moving parts
Yes, it sounds obvious to state it that way, but I've been on boats with indifferent trim and you can hear the AP grinding away to make small corrections to a boat that is largely capable of steering
itself if handled properly.