That's a tricky one, Mark. The un-answerable question, unless you happen to be a camera-shop salesman, when the answer will be weighted to the device which generates most profit!
You mention 2 full-frame cameras and that's a very good place to start: you will get from such beasts the best possible image quality per pound. The Nikon has a few more pixels on its sensor - 24.3 Mpix as against 20 Mpix on the 6D, but that is inconsequential these days. The Z5 is mirrorless, which has a few small advantages but has no real effect on picture quality which from either device will be fantaastic, so nothing to choose between them there. Lens prices are very similar for each device. The Z5 might have the edge on auto-focusing performance but that matters only according to the intended subject matter: sports? Give me superb AI tracking. Portraits? Eye tracking is useful. Landscape? Forget auto focus - you'll be using manual! The sensors in the majority of Nikon cameras are made by Sony and they are top notch: Canon uses Sony sensors in some of their small compacts but all their SLRs contain home-brew sensors: they are also top-notch. There are many fervent evangelists who argue about which is best, citing features, benefits and opinon. So, which to get? There'll be no noticeable difference in output from either when printed or on the screen so hold each in your hand, take a few pictures with them and buy the one that feels and works best for you. Cop out? Maybe ...

So much for the impartial stuff. I am (and since 1973 always have been) a Canon user and I am so comfortable with the way my picture-box ergonomics work for me that I would never change. Un-impartially, with an ojective slant - I think Canon boxes have the edge in the usability column, especially with regard to 'surface' buttons and menu operations. Oh, and - if it bothers you - Canon lens attach by rotating clockwise whereas Nikon do this anti-clockwise. As I say, if it bothers you.....
Ultimately, you will get fantastic pictures with either device. For both, expect to be spending some money on good lenses - the kit lenses that come with such devices are ok-ish but you-ll soon see the need for better.
Finally, I think that there's someone inthe club who has a 6D and lenses for sale - shall I enquire for you?