I'm not sure of the differences, although I know they are pretty different... the 94 will look quite a bit different from the 96.
As far as keeping up with a kx125... it all depends on the rider. The kdx's would be much easier to ride through the trails, that's for sure. And if I had to put money on it, I'd say that the same rider could go through trails faster on the kdx because of the different power delivery, which is designed for trails riding (vs the MX track, which is where the KX would dominate)