Remember that the RAF use specially selected instructors to train specially selected students. For the average hours building instructor teaching Joe Bloggs who walked off the street keen to fly I would say use a simple type and convert when the PPL is completed
Excellent advice, my military training was in a T-34 (retractable, variable pitch prop), solo was on the twelfth flight after fifteen hours, same instructor all the way through, twelfth flight was a check flight by an instructor who had not previously flown with you. A feature of the system was you were scheduled by the number of flights ie if you didn't go solo on your twelfth flight you were required to front a board to see if you were up for remedial training or whether they said goodbye.