I have a KX250 and my brother has a KDX200. On the trails, the bikes are pretty compatible. I have been riding for the last 16 years, and have no problem with the KX250 on the trails. My brother has only been riding for the last couple of years, and he rides that KDX better than I ride my KX on the trails. I used to have a KDX175, and that thing hauled butt on the trails, just like my brother's 200.
If you are looking for a bike that is built stricltly for trails, then I would go with the KDX200, the MX bikes have a lot of power that can be a little more difficult on the trails. The reason why is, the KDXs have a heavier flywheel, therefore you don't have the pull from the bike when you hit power band. That power band pull can throw you off the trails at times.
If you do decide to go with the MX bike, you'll have to feather the clutch through the trails a lot more than a KDX.
The MX bikes (such as my KX250) will blow the snot off a KDX200 on an open field. My brother and I raced through the woods and through an open field. I did all I could to pass him on the trails but he was just too fast. Once we got to the open fields, I passed him shifting through the gears. I thought something was wrong with his bike at first, but he said he was giving it all he had.
Good luck with your choice. Personally, I would stick with the MX bike, because you'll want the speed when you need it. And feathering the clutch can do wonders, especially once you learn to control the power band.