My son also came off XR100. The YZ has made a great bike for him. They aren't low end grunt bikes, but he has been in several races with it and has done well. One race in South Carolina, he raced part of our course (the last 18 mile section) which was very tough. Actually, I had more trouble in that section than any. I origially looked at the RM, but when I got ready to buy, I couldn't find one close by, so I went with the YZ. I have read that the RM's and KX's have a little more bottom, but I have never riddden either. In my opinion, it makes a great woods bike (that's all we ride). May be a little harder to ride in certain terrain, but he took to it like it was natural. He has been able to climb any hill or go anywhere I have been and stays right on my tail. Just recently I changed the rear sprocket up one to a 49 tooth and dropped the front by one to 13 tooth. That is all the mods I have done. I am having trouble getting this thing NOT to spooge like crazy. Played with jetting again this afternoon and will test run tomorrow. To sum it up, I don't think any of the 85's would be a bad choice. The YZ has plenty of mid and top.
With my 165 lb on the seat, I can hardly keep front wheel down through fourth gear. I was getting on it pretty hard though. I know he will never ride the XR again. Guess I'm gonna have to sell it.