KX 85 vs. KX 100

I'm trying to decide which KX to upgrade to for my 9 year old. He currently rides a 65. I would typically be looking for an 85 but the 100 has bigger wheels and I can put a "lowering link" on it that will make the seat height slightly lower than the stock 85. That way, seat height won't be an issue. I am wondering about the power delivery. I'm guessing the 100 will have more low end (but probably more "hit" too). He rides primarily in the desert but he also goes to the track now and again to ride MX. Anybody have opinions on how much more stout the motor on the KX 100 is compared to the 85 ? He doesn't really need more motor I suppose but the 100 would probably last us a couple years longer as he grows and the additional low end would be helpful in the desert, as would the larger wheels. What do you think ?

Actually, the motor really isn't the issue, it's the height. The 65 is too short for him. There is an outfit (Koubalink) that makes lowering links but nobody seems to make links to raise the bike. We've already bulit a taller seat, which helped, but only for awhile. You know how kids grow sometimes. So, the 85 is the next logical step up in seat and handlebar height. He's a pretty level headed rider too so I don't worry too much about him riding over his head even if he ends up with more power than he needs. He's ridden a friends 85 and he did real well.
Thanks for the reply.
Don