The question you should really ask yourself is "what is my budget", because the 180mm Super sells at significantly higher prices. The lens was radically re-designed and it is slightly sharper and also more compact than its predecessor. It is considered the "state of the art" of Mamiya's lens designing ability for the C system. Personally, I repute it an excellent portrait lens, and (maybe only coincidentally) I have taken with it possibly the best portraits of my life. It is more recent, and in this sense its shutter may be more reliable than that of its predecessors (less years, less operational cycles, less hands that used it, and so on). Should you decide to purchase the "super", check twice that it comes complete with the chromed reinforcement rings on the front threads: the metal is as thin as tinfoil and it would be instantly damaged if the filter threads are not protected in one way or another.
The "standard" 180mm, however, with its long-focus classic triplet derived design, will also qualify as a superb portrait lens. As all lenses with few elements, it has an excellent contrast.
Unless you happen to like over-compressed planes, in my opinion the 250mm is less advisable as a portrait lens for the C system: the camera+lens assembly will get bulky and unbalanced, the focal is too long and more difficult to use handheld, and it has a typical telephoto design which I tend not to use for portraits (it will also have to be cocked manually each time, but this is not a real issue).