Blimey, talk about a heated debate...
A ducted fan in front of the radiator will impede flow when the fans not running, and you are moving along at a reasonable speed, as you are blocking certain areas of the rad with the mechanism. However, the ducting will create a funnel effect, meaning that a high pressure area will be created in front of the rad in the duct, which will escape through the radiator to the low pressure area in the engine compartment.
An unducted fan will do the same, but to a lesser extent as there is no cowl to block the rad so much, and air will try and escape around the sides of the rad, down underneath the chassis rail etc
The same would apply if the fan was behind the rad in both cases, but to a lesser extent, as the airflow from the motion of the vehicle would push the air through the rad, but the exit of the air from the back of the rad will be impeded.
When the fan is running, a ducted fan is way more efficient as you create a high pressure area between the fan and the radiator, which forces the air to go through it.
For low speeds, either in front or behind should be fine, as long as the fan can shift enough air through the radiator. At higher speed, where the fan is in relation to the rad is irrelevant, as the speed of air from vehicle speed is way in excess of what the fan could shift, as long as your fan and cowling don't block off so much of the rad that would mean the air couldn't get to it..
