The Kx100 is a better trail bike than the yz85. It is slower but works better for the slower sections. The yz85 has a frame and layout that favors taller riders and you are already plenty tall enough for it. It is a nice simple engine, and if you can find one with the bigger wheels you will get a bigger thrill out of it.
On the downside the 85 won't go as long between rebuilds and around here, finding the big wheel versions are tough. If I ever get around to it I am going to finish a couple of yz85 projects (bigbore kitted ones with timing retarded to be more trail friendly). Those are both small wheel versions.
For the parts of nevada I have ridden in (virtually all of it actually) I would prefer the big wheel version of the yamaha to the kx100 to close.