I have to agree with those who say it depends upon the student. Some people will just do better with one or the other, and some will have physical limitations that prevent them from using one or the other. Example; my wife's friend has tiny hands with short fingers and almost zero finger strength. Not only can she not reach the trigger of a K-frame without holding the gun awkwardly, she doesn't have the finger strength to pull the trigger (or most DA triggers for that matter). She also has great difficulty in racking the slides of many compact autos, even using the proper techniques. She does well with my Browning Hi-Power, can handle an XD9/40, and has recently fallen in love with my 3913 due to the thin grip, but with her poor hand/wrist/arm strength she will start limp-wristing the guns after about 50-75 rounds as she fatigues.
If a revolver is suitable for the student, then by all means use a revolver to instruct them. Shooting DA revolvers improved my ability to shoot semi-autos, and they are great for learning trigger control. But if your student is my wife's friend, the revolver won't work for her.

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