The newer one has a square bellows allowing a three lens turret. The older one has tapered bellows. Otherwise, essentially no difference. Both are rock solid and very usable. (Right now I don't remember which is which.)
the square bellows is easier to align than the tapered. the newer lens stage allows you to align front & back, left to right. the older models cannot be aligned at a certain point--might not be a problem, just depends on the age and condition of the enlarger. you can upgrade a MXII to MXT with the converison kit for the turret mount. basically a new lower bellows and lens stage.