what I have slowly come to realise is that the old way of thinking - that the 'Camera' was what you bought (i.e. the body) and then added backs, lenses, accessories, to make your system is the wrong model today.
what we should be thinking is that you buy the digital back, and add the rest around it, as the accessories, including the camera...
It's the most expensive item, and all the rest falls into place around it - thus you can buy a Phase back, and then add/ use it with- a contax/hassie/mamiya/phase body+ lens, or put it on something like an Alpa/ Cambo, or onto your Sinar/ Linhof view camera with an adapter. its the BACK that is central to this.
Thus think first of the digital back and how many options it gives you to work in different ways, with different bodies for different purposes, and that gives you your answer. So if you only shoot in the studio with a 'blad type camera then their (closed) system is very good, but if you ever think you might want to use the back on a view camera, or on an Alpa for extreme wide work / shift, or on another type of body (Mamiya, Contax...) then you have to go with that aspect and invest in an unlocked system.