Easy choice!
Kx 100 most definately should be your choice.
the kx 100 would be great for 5 and a half feet, hell I'm 5"11 and I can still have a blast on one of those little suckers even though i could basically break the bike haha along with the 85.
Kx 100 has a bigger wheel base, and just overall is bigger, not tons bigger like a 125, just a bit.
I have always wanted a 100 myself just to mess around on, when i'm not doing that serious of riding.
Kx 100 has more power than the 85 so keep that in mind too. If you love power then go for the 100 or even buy a 125?
I don't know if you are a beginner or not, or what bike you are coming off of.
If you are looking to race, I don't know what the kx 100 classifies in.
one more thing, if you are riding a kx 100 in the trails, It will have more bottem end then the 85, so it will not stall out as much.
Good luck!