Í'm no guruh on this topic and stand to be corrected by those with more knowledge, but I think the difference between models like the South Atlantic and the three mentioned above are the type of linkage between the top arm (with the vane on it) and the bottom arm (with blade that goes in H20); my understanding is that the Seafeather, Windpilot models and Flemming all have proper gears that connect the two arms, so you get a 1 : 2 reduction giving you more power transfer from the upper arm to the lower arm.
My understanding is that The Atlantic and some other models I don't think have gears between the two arms like the three models mentioned above. I think they have a direct connection rod providing a 1 : 1 connection so the wind vain isn't able to impart as much power to the oar. I expect this is what makes them cheaper. It's likely that the gearing on the Seafeather/Windpilot/Flemming is more expensive to put together than a connecting rod. There's the Holland Windvane which falls into this category. It's a kit that you buy costing about (very roughly) half the amount of a proper bought one. Involves a lot of measuring, drilling etc. In pratice I've no idea if these 1:1 designs are any less effective. There's no shortage of people using them who are effusive in their praise.
Regards Claverton