Thank you all again for continuing to add your insights and wisdom to the question I posed that started this thread. I have learned a great deal from you on what you paid attention to such that you made your decisions the way you did. I must also say that I have learned something very important from all of you and that it is a bit of a surprise to me: the importance of a trusting relationship with the person/company selling you the digital back.
While each of us may have our reasons for agreeing with this last statement, mine is especially important to me in that I live in a place in the middle of Wisconsin that has no dealers who work with the P21+ (which is, again, what I'm leaning to quite strongly) within a half-day drive from me, the closest is in Chicago. The photographic work I do does not particularly call for speed in image capture (most of it is what I'd call fine art nature photography), and I'm rarely on a tight deadline schedule for when something needs to be shot; my deadlines tend to be on the print side, my clients need x number of prints by such and such date. The equipment I have used to date has all been purchased from dealers via the internet with whom I have no personal relationship. BUT, from what I am reading from several of you on the MFDB issue, relationship with the dealer might just be issue #1.
Prior to this thread, I had myself in the frame of mind that (like Canon or Nikon DSLRs) all of these backs come off of an assembly line where they are put together with exacting specifications, processes, and tolerances, and that, therefore, if a back passes inspection in the factory, it's highly likely that the item will work as designed. Sometimes things can still go very wrong (like the M8 problems that have been mostly fixed), I think maybe I've lived a mostly charmed life in that I've had very few episodes with equipment (cameras and otherwise), and when I did, the experiences of getting the equipment serviced went pretty well. What I've learned here though is that this is most definetly maybe not the case, that sometimes backs can indeed be finicky, or the fit between the back and the camera body can be finicky, or whatever it is that can go less than optimally, and that therefore, a key criterion for the decision is the reliability and responsiveness of the company from whom I purchase the back to begin with...especially because I don't have a company to work with that's just down the road. So, a great lesson. Thank you.
