IMO, the two hander is easier to learn and easier on the body. I converted to a two hander, from a pretty good one hander, due to elbow problems. I also believe two handers are more powerful and consistent, on the average.
Before anyone starts flaming me let me state that when it comes to playing a match, I believe one handed players may well have an advantage: more variety. They usually have better slices, more feel for backhanded touch shots, and volley better - on the average, of course. I remember that during my 1HBH days, I used to run around my backhand a whole lot more, and hit inside out. Now I don't as much, since I like my two hander... I wonder if it's the same with higher level players (I think so, but that's just IMO). Heck, one handed players may be hitting lots more forehands than two handers and reaping the benefits, for all I know.
So, it's a great question as to what coaches should do. I think it's perfectly okay for a coach to start a kid with the one handed backhand, probably with more emphasis on the slice to begin with, and teach them to topspin when the arm and stabilizer muscles are conditioned enough to handle it. And when the kid wins a grand slam, the coach can say, "See, I taught him that!"