I've owned both a KDX200 and a KX125. The KDX was superior in the woods, but the KX was great everyehere else. The KX125 is a great handling bike and will teach you how to extract the most out of the bike, and yourself. If the woods you ride don't have slow hills and large rocks, doubtful since you live in PA, the KX will be fine in the woods too.
Reliability, about the same but the KX will need top ends sooner, and you will use the clutch on the 125. I never used the clutch on the KDX.