As always it's a YMMV situation and maybe it's just me romantise-ing but I remember seeing in wonderment how the 58mm F1.4 could capture a plug socket wide open and somehow make it look as if I'd produced some piece of artwork whilst at the same time retaining wonderful colour and sharpness like nothing I'd seen before. It wasn't the D800e because I was getting same/ similar results adapted out of my XT1. Could it have been those same classic design/ abberration issues causing this?
Anyway, this 55mm is practically as good as native when adapted to a Z6; choosing between this and the 50/1 I find impossible.
I think pricing will help you decide in that respect - the f/1 lens will be in the same ballpark as the respective M mount lens - maybe up to twice the price of the f/1.2 lens. I actually expect the new 55mm to come in at a lower price than the M (or Z) mount 50mm f/1.2 lenses ... However, I could be wrong about all this; I sure hope they've improved on the optical performance overall.
In the meantime, I really hope the 40mm f/1.2 proves worthy - I'm truely intrigued by its (comparatively) diminuitive size and affordability in this context. That said, it took me a while to warm to the merits of the 50mm f/1.2 for M mount and to learn how to use it to its best potential. I've become pretty picky once lenses exceed a certain price range - to the point, I couldn't put up with the 40mm f/1.2 being mediocre - capricious, yes, it's a small f/1.2 lens, after all, but not weak to the point of being limiting in certain respects, especially after having shot with a so-called "weak" lens for the last couple of weeks that's much, much better (i.e. more usable) than its modest price would suggest. After watching and reading quite a few reviews, I'm positive that I won't be disappointed by the 40mm, but we'll see.
Longer version: I actually just played around with the old Nikon 50mm f/1.4 AF (non-D!) and the Nokton 58mm f/1.4 on the D750 for a bit, and to sum up my impressions, I wouldn't use both lenses confidently near their shared MFD of 0.45m; both become much better beyond 0.8m or so. The Nokton actually beats the Nikon in most respects, especially contrast and peak sharpness. However, the Nokton 50mm f/1.2 for M mount, while not perfect, is clearly better at 0.7m wide open than both the older f/1.4 lenses - one can argue that that's not that much of a feat, considering its price; I still think it's pretty amazing because it's hardly bigger than the 58mm f/1.4. What is *really* astonishing though is that the tiny 7Artisans 35mm f/1.2 II, a lens that clearly suffers from visible spherical aberration wide open, outdoes both the F mount lenses at its MFD of only 0.28m when it comes to core sharpness; it's still pretty bad, mind you - its weaknesses are many, especially very low contrast at its MFD (it's a super-basic double-Gauss design - almost 100 years old!), and it doesn't improve as much at longer distances. In short, for a cheap "character" lens, the 7Artisans performs quite well, for premium primes, both the old Nikon 50mm f/1.4 (which is based on an optical formula from the early 1960s!) and the still current Nokton 58mm f/1.4 show optical flaws I'm not too happy about. The Nokton 50mm f/1.2 does have some quirks, but they appear minor by comparison - but then, it's a much more expensive lens. But it *is* a lens I can rely on.
The 40mm f/1.2 is said to be only slightly less convincing than the 50mm f/1.2 - so that's what I'm banking on: I may get most of the performance of the M mount lens for two thirds of the price *and* with a much shorter MFD, so I hope I can maximise the benefit of a super-fast lens. It remains to be seen how well (or badly) the 40mm performs at close range, though.
(Sorry, this became much longer than initially intended ...)
M.