I recently faced the same dilema. I ended up with the 85. My son was stepping up from the KX 65 and I figured the 100 would give him more room to grow. You can get a lowering link that makes either bike 1 3/4 inches shorter. The 100 has bigger wheels so it will handle the bumps and sand better and I hear it has more low end than the 85. My problem was, my son likes to race MX about once a month and the 100's have to race the Supermini class. At my local track they race the supermini's with the 125 and 250 beginners. Not a safe place for my 9 year old. Anyway, we bought the 85 put the lowering link on it and he LOVES it. It has good low end and mid range and handles good too. If your brother is tall enough, and he doesn't intend to race MX, consider the 100 (and lowering kit if necessary). It will last him longer and it's probably a better bike for trail riding. It's hard to go wrong with either bike.