Among the more anticipated announcements for this camera was the fact that Nikon would be introducing a new lens mount. The Nikkor Z-Mount is an evolution of the existing Nikon F Mount and, like the Nikon Z-Mount for their 24-70mm f/2.8 lens, it's a completely new design for the brand. It's also, of course, Nikon's first attempt at a full frame, full frame DSLR lens mount.
Both high frame rate and 4K modes have 60fps to 120fps options. The 120fps mode is just video mode, not 4K. In 4K, there is an option for maxing out at 900K, but that's only at 30fps and not from full frame.
There are some interesting features. The Z5 can capture up to 16 layers of RAW, but only at 24fps (and only in the camera). That's a limited amount of information, so you'd still have to combine them with other cameras and software to get the most out of it. The Z5 can also record in 4K UHD at 24 or 30 fps internally, but there's no way to export it in a usable way because it's being recorded at the full frame width (or DX frame area) and not cropped. That's why you can't crop it out and use it. At least, not at the moment.
If you want a real-world comparison, go with the Z6. If you're looking for a fast professional camera in a mirrorless package, you should at least consider the Z7. If you're looking for the best full-frame mirrorless you can buy right now, go with the Z6. But if you're looking to buy a really good compact camera now, I'd give serious consideration to the Z7. But I'd also not ignore the older Z6 for its low-light performance.
Let's look at the Z6's dynamic range. At ISO 50, the Z6 is about 18.2 EV (Exposure Value) behind the Z7. At ISO 400, the Z6 is only 7.2 EV behind the Z7. This makes the Z6 the best in that regard. While the Z5 is significantly better than most consumer-grade DSLRs, it's still coming up a bit short at the top end. And it's not the best in low light performance, either.
My Z6 is not the best mirrorless on the market. At best, it's slightly better than the Panasonic GH5, which is itself better than the Sony A7R III. And it really does come up short when it comes to high ISO performance. It's not the best mirrorless video-capable, either. But it's the best Nikon has to offer. And that's really what matters.
We also know the D750 can produce really good looking images, up to ISO 3200. (I'm not sure if it can go to 6400, or how much of a difference that makes.) I do need to try it, so I'll let you know in a few weeks.