dsut4392 said:
1) _all else being equal_, would you like your sensor to have more or less DR?
2) _all else being equal_, would you like your camera body to be smaller and lighter or bigger and heavier [noting you can always bolt a grip onto a small camera, but you can't make a big camera any smaller]?
3) _all else being equal_, would you rather be able to adapt any lens to your camera body, or be limited to lenses with EF or longer flange focal distance? [On this latter point, your statement of "it also changes the way the optics _must be_ designed" is a fallacy, as optics designed for a longer flange focal distance work equally well (with an adapter) on a short flange camera, assuming the throat of the mount is wide enough. The differences in optical performance between some native lenses vs adapted (for instance as observed by Roger Cicala over at lensrentals) are due not to the flange focal distance but the different cover glass thickness assumed by the lens designer. Shorter flange distance _allows_ changes in optical design, but doesn't force them].
4)_all else being equal_ would you like the option of some lenses that are more compact than the same focal length and aperture in EF mount?
1) All things being equal, I want as much DR as possible. If the sensor can cover a hundred trillion colors from infrared to ultraviolet and record it all perfectly at a thousand frames a second, great.
2) All things being equal I want my camera as light as possible, full stop. In terms of camera size, I want my BODY appropriately sized to the LENS I will most often use. I also want the bottom of my body to be deep enough to accommodate an arca swiss plate horizontally.
3) Whether things are equal or not, I will never, ever use a lens adapter to match current lenses with current bodies. Full stop. Ain't gonna happen no matter what the benefits are. If things aren't equal, I would rather spend $20,000 on new equipment than have an adapter stuck onto my camera body. ****** that. I hope I made that part clear
4) All things aren't being equal. I want my small travel lens to be as small as possible and I am willing to give up optical quality and big aperture, and I'm willing to buy new stuff to make that happen. I want my big pro lens to be whatever size they need to be to give me the best output at "reasonable" prices. For example, a Canon vs Sony 70-200/2.8 might be a little better or a little worse, one way or the other; personally, I think they're close enough in optical quality, though I think the Canon's construction is significantly more rugged. But anyways, I won't happily pay the Sony premium on the lens. This is part of a "new lens system problem": if Canon came out with a new mount and all new shiny lenses that were slightly smaller and slightly better in every way, they'd be 50% more expensive for a long time, and the improvements would (probably) be too small to justify the price, for me.
Lens weight: frankly, if it's under 1kg, I don't care at how big or heavy it is, within reason. If it's an important lens to me (like a 70-200 and 100-400), the 1.5kg range is fine. Sure, it can be a little lighter, but whether it's 200g either way isn't going to be determinative of which I buy.
I should add:
5) All things being equal, I'd rather have decent weather seals in an expensive camera. This isn't a deal-killer for me, but if the top tier camera doesn't have this, there would have to be at least one thing that was out of this world that I wanted out of it, where there wasn't an alternative elsewhere.
6) Whether things are equal or not, battery life while I'm looking through the viewfinder is very important to me. It doesn't matter how good electronic viewfinders are, if I keep running out of battery while I'm staring down the viewfinder, the camera is junk.
7) I don't care if the camera can even record video. Since 80D and 6DII, I have never recorded even 1 second of video on any device other than my smartphone. If, one day, I want to be an amateur videographer, instead of making my $4000 camera do something it's not ideal for, I'll just go spend a bit more and buy... or rent.. an entry level professional camera rig. There's a reason that the 6 o'clock news isn't produced on a Sony A7R or a Canon or Nikon DSLR.