In my opinion it is easier to learn to fly a four channel aircraft then go to a three channel aircraft then the other way around. the flat wing will not help you out as much as a wing with dihedral but it shouldn't be too hard without it, it just might take a few flights longer. I had a friend of mine who knew how to fly three channel fairly well but when he flew a mid wing aircraft with no dihedral and the wind flipped him to knife edge he reverted to a three channel mentality hit the rudder full over and wondered why the aircraft never righted itself. I can see other instructors points about learning three channel so that you learn to use a rudder but forcing you to only learn on 3 first is not considered an optimal approach. My instructor taught me on a four channel and some of the lessons he taught me was flat turns using rudder and aielerons, cordinated turns and stall turns.