Neither is a very good choice for anyone other than an adult male looking to do some trail cruising. They are very heavy, about the same as a 450. I think they are actually heavier. If your son can get a toe or two one the ground get him a 250F. I feel that the CRF and the YZF are about equal except the YZF will usually take a crash a little better. If he simply out grow the power of the old bike get the new CR150R. There is nothing like it. I hope Yamaha comes out with a new YZ150F soon. My wife has moved behond the TTR and I really want to stay with Yams but that 150 is the best bike for her. She is only 5 feet tall so a full size is out, although she loves to take my 426 out as long as I can hold it for her to get on and off.