Coming in very late to this, but shall stick this here...
Until the firmware announcement for the Z8 today, which should address the one quibble I have with the Z8, which is in the realm of AF for studio/professional strobe environments not being as good as my D850, my thought pattern was this - just to show I'm not anti-Canon in any way.
IF I were still shooting the amount of studio work I did 10 years ago (I'm not, I'm aging, slowing down, and it's all limited personal work these days), I would have been running dual systems - a Nikon Z8 for landscape, outdoor portraiture, outdoor dance, and a Canon R5, probably the mk II, for studio work. The advantage of the Canon AF, for the specific use case for studio work, was the reason. Obviously if the firmware addresses this issue, this concept/thought pattern would be off the table. But say it was 3 days ago, this is what I would have done. I would have had 3 Canon lenses - the excellent 28-70/2 RF, the 85/1.2 RF, and the 135/2 RF, and that would cover me for studio work.
For landscape, I find the Z8 still the best between the two for low ISO (64) shooting, even with it's slightly lower DR compared to, say, the D850 or Z7-II. Outdoors, for non-landscape work, I have no problems with the Z8 AF system, and it's an *extremely* responsive camera - more so than the R5 series, and that does matter. I also prefer it's EVF experience.
Where I will obviously have my preferences is lenses. All major manufacturers are putting out very good glass - we live in good times. But for what I do (landscape and studio and outdoor people) and what I value, Nikon is in the lead. Lenses like the 14-24/2.8S, the 24-70/2.8S, the 85/1.2S, 105/2.8S MC, and the Plena are top of the class - Canon to my mind doesn't have a match, although their 85 is quite nice of course. And at the end of the day, one marries the lenses and dates the bodies, so Nikon is always my first choice and it was just a pain that the Z8 AF didn't have the feature they just announced in the latest firmware which should instantly solve the studio strobe AF quibble.
Now, as to sharpness. Sure - if you're just shooting out of camera jpeg and that's all you're doing, yea, you might see some minor differences beyond the lenses impact on things, but it's mostly just settings. And to be a bit blunt, if you're *that* concerned about sharpness (in an era when frankly, a lot of stuff is easily professionally sharp), you should be shooting raw and being extremely good/careful at post process sharpening.
As for test sites - they don't tell the whole story, so I wouldn't make decisions on lens sharpness from them either. You'll have to gather consensus info from various subjective sources as well as a few "objective" sources. An MTF50 score, or a DXO score, is honestly nowhere near "enough" to fully and accurately describe a lenses performance envelope.
So that being said, while I am set, mostly because of semi-retirement/personal work being my primary thing now and because the Z8 gets firmware that solves the one problem, I'm still going to recommend the following approach.
- ALL camera systems these days are damned good. If you had told me, 40 years ago as a recent photo school graduate, of the quality achievable with ANY of the major players systems, I would have laughed at your face and checked you into the nearest and handiest insane asylum. We have it really, really good today.
- How YOU (not me, not the test site, not the youtuber with a bunch of viewers, no matter how much you like or don't like him) get ALONG with the usability of the camera is #1 and then some. Easily #1. We can quibble about lenses - and for me, I do, because I'm very picky, but if you don't like working with a camera, it's game over. Note that in an earlier paragraph I said there was a thought pattern I said I would have dual systems - why did I not choose Sony? I can't stand their bodies. I honestly can't get along with them. So it's game over, right there.
- The more you are a jpeg only shooter, the more the way the camera defaults it's processing to what they think is natural/correct/whatever is going to matter to you.
- Personally, I could never shoot this way, but for those who do, if you don't get along with the native output from a camera systems jpegs, and you're of an understanding that these can be changed to some degree, then again, it's game over, the brand ain't for you.
I sense that you're likely heading over to the R5 and that's cool. In summary, I'd say don't place so much thought into sensor type arguments, and no, sensor types don't matter much at all to RAW shooters with regards to sharpness, and again, most everything today is quite sharp and it's often other aspects of lens image quality that become to matter more.
Good luck, we'll miss you if you leave, but you gotta do what you gotta do!
