If you're going to do trails on a 125 MXer, I would recommend the YZ. They seem to have a wider powerband which should help on tighter trails. That said, a friend of mine rides his CR125 (narrowest powerband I've ever seen) on trails all the time, and has quite a bit of fun. It's a matter of getting used to the bike and jetting properly for smaller throttle openings. You might foul quite a few plugs at first but get the jetting down and remember to wind it out every now and then.
Another suggestion I can't help making is to skip the 2-strokes and get a YZ250F. That's what I ride and it does great for MX and tight trail riding, especially with a 50T rear sprocket.