Great test. I am coming from an Aptus II 12 R. I shoot product on a sinar P2. using a bass V mount. I am wanting to upgrade and get a MF system as well. With the release of the 50mp back, the higher iso is attractive. I can't seem to shoot my back above 100 without it starting to fall apart. All I really need is 400, and that would help a lot. I don't see much loss of detail between the 80 and the 50, besides the loss in megapixels. Do you see any reason not to downgrade to 50 megapixels?
As stated above, smaller sensor (longer effective length for any given lens) is the biggie. Dropping from 80mp to 50mp is a loss of resolution, but both are quite high in resolution for most applications. Ultimately only you can decide how much resolution you want and how much you need. Take some of your 80mp files and downsample them to 50mp; the result won't be exactly the same, but close enough to make an educated decision based off of. Alternatively contact a dealer and get relevant raw files from the Credo 50 or IQ250 so you can make your own processing and output and decide what you feel about the resolution.
The 50mp sensors struggle with movement on wider lenses, especially symmetrical lenses like those of Schneider, when used on a tech camera for architecture or interiors, especially when used at infinity. It's still very possible to use an IQ250 for those purposes, you're just limited to Rodenstock lenses and less movement than you could get with other backs, so in general it's not our 1st recommendation for those applications (landscape/architecture). In contrast, when shooting product about bread-box sized and smaller this isn't much of an issue - your focus distances mean that the lens is quite far from the sensor and the amount of movement
usually isn't as significant as with architectural/interior and it's less common to use very wide lenses. We did some fairly extensive testing for a client who exclusively shoots table top product; in his case (meaning the lenses, focus distances, and amount of movement that were typical for him) the color cast was not deemed an issue on either back and the significantly improved live view won the day.
My suggestion, which is transparently selfish, but I think a good suggestion none the less, is to work with a dealer which has done extensive testing on these questions, and communicate to them exactly what kinds of lenses, movements, and focus-distances you use and they can let you know if you're barking up the right tree. If you fall outside the "clearly okay" or "clearly a problem" ranges then arrange to do testing at their facilities, or arrange a rental through them to do testing at your own studio.
But really, for live view, you'll be amazed at how nice the USB3 based Live View is on a large monitor (or also on the back itself in the case that you're arranging items in front of the camera and don't want to crane your neck to see the monitor).