They are all very good quality. I owned an X2 for a while. As for the motors, it can get confusing. Pitster buys motors made to their specs and then calls them GPX motors. Motors can be manufactured by Lifan, Zongshen, Jialing, etc. If Pitster put any of these on their bike, they would then be called GPX motors. At any rate, if the motor is on a Pitster bike (from the factory), no need to worry. They are all good and Pitster will stand behind their product. As for the bikes, I didn't like the 10/12 wheel setup on the X2. I wanted a 12/14 setup. X2 and X5 are the same price. X4 is about $450 more. Personally, I'd buy the X5. If the boys are vertically challenged, then the X2 would be better. You can't make a bad decision with any of them.