You're 5'6", 140lbs, coming off of an XR200R. Unless you ride desert, wide open roads, etc, I don't think any 14 year old could handle a bike like that. I could see if you were 5'11" or more, weighing atleast 160lbs. What happens if you crash? An XR400R is HEAVY. I haven't ridden an XR400R, but I have ridden an XL250S, which is similiar in the weight department. I dumped it, and there was no way I could get that bike up by myself. I'm 5'9 or 10", 140lbs. Trust me, your dad is right, go for the 250, not the 400. Have fun. Also, 250s have lots of hop-up potential.