Never mind the theory. I had to turn the engine off on a Chipmunk once. I found the drag greater with the windmilling prop,(as opposed to the prop being driven at idle), and the resulting forced landing pattern had to be modified to ensure I didnt land in the undershoot. I was in regular practice at forced landings - I was an instructor on the Chippy at the time, I did many each month, so I noticed the difference.
The fairly coarse prop on the Chippy ensured that the prop did not stop until on the landing roll. I'm sure the finer examples on Cessenas/Pipers would actually stop, but I can only surmise that the drag would be similar to the Chippys', or perhaps even greater.
After a crewroom chat with my fellow instructors, we concluded that the prop driven at idle would present a smaller angle of attack and therefore less drag. (working in reverse of course.)
Why did I have to switch trhe engine off in the first place, that's a different story which I won't relate on the grounds that I will make myself look stupid!
P.S. The pattern I used was the military constant aspect procedure. Far better than the civilian one. Anyone care to comment?