Here's what I've owned and used extensively:
Square Kodak Sensor, 36.7 X 36.7, 16.8 meg., 9X9 micron pixel Backs with 1.5X lens multiplication factor:
12 bit., Kodak ProBack for a Hasselblad 555 ELD and Mamiya RZ Pro-II using a Kapture Group V to Mamiya adapter. Files directly supported by Adobe Camera RAW. ISO 100. Needed to be tethered to a Quantum battery for mobile work.
12 bit, Kodak ProBack 645C on a Contax 645. Dedicated mount. (Same ACR support as above.) ISO 100 to 400. Self contained, underslung battery.
16 bit, Imacon 96C on Hasselblad 500 series V cameras. Dedicated V mount. Needed Flexcolor Software, but could convert to DNGs for Adobe use. ISO 100 to 400 (but 400 was not very good.) Needed to be tethered to a Image Bank for mobile capture.
16 bit, Hasselblad CFV on Hasselblad on both 200 and 500 series V cameras. (Same file support as above.) Firewire 800 connections. ISO 50 to 400 (soon to be 800)
Rectangle Sensor, 16 bit, 36.7 X 49.0, 22 meg., 9X9 Micron pixel backs with 1.1X lens factor on 645 cameras:
Leaf Aptus 22 Dalsa (slightly smaller sensor Valeo upgrade), CMOS RAW files Directly supported by Adobe Camera RAW. ISO 25 to 200. Underslung battery.
Hasselblad H2D/22, Kodak sensor (which produced DNG RAW files right from the camera with no conversion of propritary software needed.) No film back useage. ISO 50 to 400. Integrated Grip battery.
Hasselblad H2D/22, Kodak sensor. Later model that reverted to use of Hasselblad's 3F file format, still allowed DNG conversions, but now also allowed use of film backs. ISO 50 to 400 (soon to be 800), Integrated Grip battery.
Rectangle Dalsa sensor, 16 bit, 36 X 48, 33 meg, 7.2 X 7.2 Micron pixels. 1.13X lens factor on 645 camera:
Leaf Aptus 75 with dedicated Mamiya mount for AFD-II, and used on an RZ Pro-II with adapter. Underslung battery.
Leaf Aptus 75s same mount (differed from above in capture speed and now was Firewire 800.) ISO 50 to 800.
Rectangle Kodak sensor, 16 bit, 33.1X 44.2, 31 meg, 6.8 X 6.8 Micron pixels with micro-lenses. 1.3X lens factor on 645 cameras.
Hasselblad H3D/31 dedicated back on H3 camera, ISO 100 to 800 (soon to be 1600), accepts film backs. Grip battery.
Rectangle Kodak sensor, 16 bit, 36.7 X 49, 6.8 X 6.8 Micron pixels, lens factor of 1.1X on 645 camera:
Hasselblad H3D/39, active cooling via fan, 2.5" LCD, grip battery, Integrated camera system, no film back use.
Hasselblad H3D-II/39, my current camera/back. Differs from above with new button configurations, heat sink cooling, and 3" HD LCD.
In addition, I've hired professional photographers in my job as a Creative Director for ad agencies. The most used digital backs in that extensive experience were Phase One P25s and P45s, distantly followed by Hasselblad H2s with 39 meg backs, and a few Multishot Hasselblad CF backs on all kinds of cameras from Hasselblad Hs and V to Contax and Mamiya to view cameras.
Sadly I cannot lay claim to seeing any Sinar backs in action. It's a hole in my direct working experience, so any conclusions I make here are sans that input.
I'm not selling anything here. I made my choice based on experience and MY needs, no one else's.
IMO, they are all so similar it's mute to argue about it. The IQ is there on all of them ... in spades!
I think for a Contax 645 the way to go is Phase One. It's like the their backs were made for that camera.
I think for Mamiya, the Leaf Aptus Backs are a perfect match. Phase One is now in league with Mamiya, so that may change dramatically. But until that solidifies, I'll stick with my experiences.
Hasselblad has gone it's own way. Either you love it or hate it. I love it ... love it enough to drop mega bucks into it and an Xact system to use the back on. I will say that categorizing it as a "studio camera" is news to me, Hasselblad H users world wide, and to Hasselblad. Enough said.
HY6.
I'm a visual person. I think Sinar industrial design needs updating. The Leaf AFi Hy6 is beautifully integrated industrial design. I DO NOT know anything first hand about the functionality of a Sinar back or their Hy6 camera system. I DO know Leaf functionality, plus their service, durability and software is excellent. I have no doubt that Sinar's is also, but I have no direct experience with it.