Having been on staff at many of the national photo magazines I have had an unusual opportunity to use nearly all the medium format cameras on the market, most for extended periods of time. If you are going to be doing studio portriat work (or any studio work) I highly recommend the Mamiya RZ67. It is a bullet proof workhorse of a camera.
i finally ended up with the Pentax 67 because it is fast and easy to use and I don't need interchangeable backs or Polaroid. I was also told by an industry insider who was asked to test Mamiya, Hasselblad and pentax lenses to see which were sharpest that Pentax lenses are the best of the 3. Everyone was surprised and the company that hired him buried the study. If you are going to do fine art, or if you are very confident in your exposure technique I can highly recommend the Pentax.
As far as I know, it is also the only medium format camera that changes from 120 to 220 film without having to buy another back or insert. Considering the fact that backs for most cameras are going in the $500 to $900 range these days, that can save a few bucks.
Good luck with your selection. Let us know how it works out.