I have owned both a KDX and currently a KTM 250, so I can "kinda" give you some input (even though mine is a 250) The KDX is a great trail bike, and can be ridden moderately hard. It will start every time and never leave you stranded. I had one for 8 years and never had a mechanical failure. I am not sure I would have taken the KDX on a MX track unless it was just playing around, for serious racing? NO WAY! The KTM is a great motorcycle also, but it is a more serious race bike, that can be ridden as aggressively as you can push it. I have only had mine 3 months so I cant comment on reliability. I would feel as comfortable on the trail as the track with this bike. If you feel that the YZ felt heavy, then you wont want the KDX either. At your size, I would think that any newer 250 would be just right, but the KTM 200 has the 125 frame so it is lighter and more flickable through the trees. Do a search, there is a wealth of info on the KTM 200, most people say it is the funnest bike they have ever ridden, regardless of their size or riding ability.