The reason for the fans is to cool the radiator so my opinion is that you use the one in the leaving the radiator tank. The likely reason there are two locations is they are copying the original factory layout without knowing why. The original radiator had a vertical baffle in the tank so that coolant flowed across the front and then back across the back. This meant that both sensors were reading the leaving coolant temperature. Somewhere in the 72 area ford determined that it would be more efficient for the flow to be across the bottom and return back across the top. They changed the baffle from vertical to horizontal to make this happen and issued a tech bulletin for dealers to have this done to earlier cars but did not address the fact that now one sensor is measuring entering coolant temperature and the other leaving coolant temperature.
I assume that the aftermarket manufacturers have just copied the original layout without knowing why it was done the way it was.