My sister rides an RM 85 and my girl friend rides a yz 85. The RM is much much easier to ride than the yz. Rm has a power valve and it certainly makes a difference. The YZ hits really hard and is a bit much for a young rider. However if your son is a talented rider and can keep it pinned the yz would be the way to go. If he rides in the bottom or mid-range at all the rm is the way to go. I ended up putting a flywheel weight, a size down front sprocket and had the cylinder ported on the yz to make it user friendly. This is not saying the RM is weak on top, it just has a wider powerband and can be lugged more than the yz. My good friend (adult female) has a cr 85 expert and it was in between the two, hits harder than the rm but not as hard as the yz, bottom and mid was better than the yz but not as strong as the rm. I rode a kx 100 when I was a kid but I can't tell you the power characteristics, I was nothing special, top 5 rider in supermini. But I climbed lots of hills in hillsville with my dad and managed to do all the doubles and triples at the local tracks with it. I don't remember being scared of the power so it must have been pretty smooth.