Very well put Peter. There is little in what you said that I could disagree with. I will add a couple of things though for Jim's benefit making a very hard decision. And also should disclose for the record that I presently own a Jenoptik M11 in Hassy V mount, and a Leaf Aptus 75S, in Contax 645 mount.
I have gone through most everything myself in trying to make my own buying decisions in the past. So Jim, my heart goes out to you and everyone else in your position. It is not an easy decision, nor is all of the information you probably want available without a "bum's rush" sales pitch thrown in whether or not you really wanted to hear it. I had a real nightmare several years ago when I bought my first back. There wasn't information to be had, and even fewer opportunities to actually feel or handle the backs I was basing my decision to spend as much for as I would a new car that I could at least see, touch, and test drive at my local dealer. Things are better today for sure, especially given the great resources like this one Jack and Guy are providing, and the greater number of we working professionals that both own the gear, and use it in daily commerce.
First off, let me say that in my opinion, today the major "differences" with all of the MF backs have converged to the point that they are all very good at what they are designed to do best. None of them would I call "dogs" today. Imacon/Blad finally got rid of the "Mellow Yellows" syndrome, and PhaseOne has dialed back the overblown color punch that existed when I bought my first Leaf back. So any of them are going to be pretty close really to each other. I say this as I duck the flying rocks that making that statement here probably exposes myself to.
My personal preference is towards Dalsa sensors, as I do value and see a benefit for my own work from having a true full 16 bit A/D converter in the color gamut & tonal range. But the Kodak sensor backs do work far better for longer exposures and at about a full stop or possibly more of high ISO shooting. Each manufacturer implements the sensors in different ways, with differences in the workflow and controls. Probably the strongest MF post processing software would presently be PhaseOne's Capture One, but the latest information I get on the new Hasselblad software does look good. Leaf brings up the rear by a long way with their Leaf Capture 11, but does enjoy very wide support in the other software products available to the market. In the end, I guess your decision will have to be made based upon your available budget, or how well you get along with your bank loan officer, and upon your own specific needs & wants. Your subject matter also will be a large factor too in deciding between the subtle differences in the backs. As Peter already pointed out, the Alpa will accept any of the backs, in almost every mount you can imagine. Andre & Co have done a really great job in making sure of this. They even have their mount ready now for the new Leaf AFi/Sinar Hy6 body, and it isn't even shipping to dealers yet. Great camera system by the way, and one that I am likely to be adopting myself at some point in the future. So Jim, what are you going to be using this new MF system to shoot? Maybe we can make some suggestions on what you should consider looking at at least.