I have reviewed both of these cameras for uwpmag.com and this is my take as a still photographer. First these cameras are Nikons first jump into the full frame mirrorless market. I reviewed the Z7 as a pre-DEMA teaser for the Nov/Dec 2018 issue #105. At the time the only lens I used was the Nikon 60mm F/2.8 macro with the FTZ adapter and Nauticam's MWL-1 and SMC-2 C/U lenses which were also about to debut at the DEMA show. At the time the auto focus was not up to par but that has been corrected with firmware updates. With the Z6 (M/A 2020 issue #113) review all of the introductory shortcomings has been corrected and I also had the opportunity to use the excellent Nikon Z 14-30mm F/4 S and the also outstanding Nikon 8-15mm Fisheye with FTZ adapter.
As stated above both cameras are very similar using the same frame and excellent built quality Nikon is known for. Both cameras fit into the same housings without any modification. This reduces production costs and benefits the U/W photographers like myself who carry the high MP camera while traveling and use the low MP camera as a backup and surface camera. At the $2000.00 and $3400.00 price points I would have expected Nikon to offer a little more on the high end. Both have the same EVF I would have expected the higher 576,0000 rather than the 369,0000. Both have a 1/200th flash sync speed, I would have expected 1/250th for $3400.00. Both have a single card slot which they got slammed for on release by terestrial reviewers and both use in-lens stabilization which only works with the limited Z lens line. I apologize if this issue has been corrected in a FTZ firmware update.
Most U/W photographers don't use the higher frame rates of the Z6 but it is always nice to have and I would rather have the Z7 ISO range on the bottom end rather than the top 204800 ISO on the Z6. Most of my U/W work is done in the ISO 50-1600 range so the Z7 suites me in this regard. Most U/W photographers will be more than happy with the Z6 and if you find the need for more MP's as you expand your horizons you can always add the Z7.
The biggest issue is how much are you willing to spend because the housing, ports, extensions, strobes, arms, lenses, gears and more will all be the same for both cameras making the overall difference an additional $1400.00 for the Z7.
If you wish to read my reviews they can be found in the back issues at uwpmag.com, these are free PDF downloads.