I own both (the f/4 via a Z6 kit and the f/2.8 via a sale). I started out with the f/4 and liked it a lot for field use, with the caveat that I like having wider apertures available. (I'd been spoiled by the f/1.8 primes I'd been using on my D750 on previous trips). I haven't posted the f/4 photos here yet.
Then I got the f/2.8 and took a lot of photos, none of which I've posted here yet. This is my first f/2.8 midrange autofocus zoom. The image quality is excellent, having f/2.8 available is wonderful, but the lens is much larger and heavier aside from the high price.
I really comes down to the very old photographic tradeoff of technical capability vs. size, weight, and cost. Both lenses have excellent image quality for their class, with some calling the f/2.8 the best in its class. The f/2.8 is reputed to be 'better' than the f/4, though I haven't tried to compare the lenses directly.
I think the real issue is, do you want a smaller, lighter, more practical lens, of excellent quality for its class, that costs nothing because you already have it?
Or do you want to spend a lot of money on a lens that's even better, that gives you more control of depth of field (and light, under marginal lighting conditions), but which is heavy and bulky?
For me the choice of an f/2.8 midrange zoom was always somewhat unattractive due to cost, size, weight, and optical compromises (the last up through the G version). Why would I want to lug around that big lens, after paying over $2K for it, when I really want even wider apertures and better corners, as well as a longer long end?
On the other hand, my set of three f/1.8 primes*, however nice the available apertures, always involved some photographic friction caused by lens changes. Also, foot zooming doesn't always work in a landscape.
The 24-70 f/2.8 Z finally broke down my resistance and I sprang for it. It's rewarded me nicely around the neighborhood though I haven't gone on any trips with it yet. I may use it as my standard lens on some future trips because I've found I really like having f/2.8 and not having to change lenses. I think I can manage the extra size and weight when placed on a Z6.
Oh, tradeoffs....
In the end only you can answer your question. I hope my thoughts will help yours to gel. I'll try to post some photos taken with each lens shortly, but again I don't have any direct comparisons or even anything similar.
*Nikon 24/1.8 AFS, Tamron 45/1.8, Nikon 85/1.8 AFS.
