If you ever have plans of ever racing you better go with the 2 stroke as they are a great piece. I would leave the 4 strokes to the next guy. There is so many parts you can get for the 2 stroke at a lower price than the 4 stroke. First of all there is a big difference in the 2 stroke and the 4 stroke the 2 stroke compares with the 125 4 stroke I hope this helps you out. YOUR BEST BET IS THE 2 STROKE all means.
Even though you are not racing the 2 stroke is the way to go, he will want more power eventually and you can do so much more to the 90 2 stroke and for alot less money than the 90 4 stroke with much better performance.