Sensor first. The Kodak 39 chip is a good chip and with Phase's implementation you can shoot up to an hour, but do note you will then need to shoot another one hour exposure for a black frame to calculate out noise specific to that exposure.
The 40 is also a Kodak but microlensed, this means you get better ISO performance than the 39 but may cause problems on tech cameras (not that the OP asked about that)
The 40 is a much newer generation chip I would guess 5 years newer than the 39 (Steve is this right?)
I have compared the 40 with the 31 and it is much better (I own a 31) apparently the 40 has better dies making for better colours.
I think the Kodak KAF-40000 is about 4 years newer than the KAF-39000, but who's quibbling.
The great pity is that while Hasselblad has the better sensor in the KAF-40000 (higher signal Q.E., lower readnoise, and in particular, 4-5 times lower darknoise), they are not fully exploiting it. Limiting the longest exposure to 256 seconds is a mistake (and why is it even necessary
to have this limit?). If this sensor was in the hands of PhaseOne, their Xpose+ long exposure technology would make multi-hour exposures possible.
Great pity #2 is that PhaseOne, like Leaf and Sinar, now seem wedded to Dalsa; and I fear there won't ever be a KAF-40000 (or better) long-exposure capable MF sensor that I can put behind my Mamiya 645 glass.
From what I've seen and read, Pentax have delivered the ability to take clean, arbitrarily long exposures with the KAF-40000 in their 645D - well done Pentax! But M645 lenses can't be adapted to the P645 and still focus to infinity.
What's so great about Mamiya lenses? Can't I just switch platform to get the sensor I want? Well, I don't think it was mentioned in this thread, but for some shooters the big downside of the Hasselblad system is the limited range and speed of the lenses. There's no full-frame 645 fisheye, no f2.8 wideangles, no f2.8 APO teles, no lens longer than 300mm, no zoom beyond 110mm, no soft focus. Adaptable lenses are limited to Hasselblad V ones with leaf shutter; they are even slower than the H lenses, but they do at least extend the range to 500mm and 30mm (fisheye on 6x6). Pentax also falls short in the lens stakes, but much less so. Many ranges of lenses can be adapted to the Pentax, but even more can be adapted to the Mamiya (including, indeed, the Pentax ones!)
Right, mission accomplished - I've sent this thread into a "Who's going to give Mamiya users a 6-micron long exposure DB