I shot the x1d for about six months/1,000 images and now the d850 for about a year. I shoot mostly static abstract subjects, some semi-macro. My target is large prints, generally 3 feet or greater in whichever is the smaller dimension. I sometimes shoot on tripod, but a lot of the time I am constrained to handheld. About half the time I am focus-stacking because of DOF limitations or I will shoot a two- to three-image pano for more resolution. I never do both, because I am lazy and the success rate is too low.
My gold standard for best imaging for my type of imaging is, by far, Hasselblad multishot, but I have never personally shot that camera because of cost and hassle factor.
I shoot the d850 with Zeiss 100/2 or 135/2 almost exclusively, and likewise the XCD 90mm on x1d.
My opinion is that they are really quite close. My XCD 90 was a little bit better the Zeiss 100, but then the 135 was a tiny bit better still. The lens differences are small, and mostly apply wide open where I don't use the much, and yet they are usually what makes it so I can see a small difference. After post-processing, it is very difficult to see a difference.
The 4:3 ratio at 50MP does yield more information for a squarer-aspect image, of course. My images tend to be squarer, so this yields a small but noticeable advantage.
On the other hand, to me the d850 feels much more responsive and has better haptics/ergonomics. The MF helicoid is also my favorite, although the xcd focus by wire is really quite good. I have more confidence in d850 in inclement conditions, and I can have two of them for the same price, and I generally try to carry two bodies on trips.
I would happily shoot with either and not regret it, but I think the d850 is the clear winner for the kind of stuff I like to shoot. I'll happily move back to an x2d or whatever else when the time comes, though. And if anyone offers a multishot implementation on a modern sensor, well, that would be the bee's knees.
