Yes, it's disingenuous for me to say they're 'the same'; more of an evolution, like a 2012 Golf GTi vs a 2018 model. But in real world use, I found the D600 to be every bit as good as the Z6 at lower ISO settings. Those 24Mp sensors were fantastic all-rounders, and had great dynamic range.
I wasn't at all sold on the new Z system when it was announced; I bought a Z6 as a replacement for a D3300 I'd been using as a 'travel' camera, because it was smaller and lighter than the D600. With the 24-70 kit lens, it made a great little package for travelling and that. But it very quickly grew on me, and I found the viewfinder to be far better than I'd expected or experienced previously. I started using it at gigs, and it quickly took over as my 'main' camera. The D600 is better ergonomically, but the low light performance of the Z6 beats it hands down, and the D600 is already a great low light performer. The silent shutter is also a major boon at times when I need to be quiet. I was covering one event where another photographer was ordered to stop shooting by organisers, because of the 'machine gun noise' his EOS 5D was making (a speaking event which was being recorded). I was able to continue taking pictures, because of the silent shutter mode. Such things, you don't know are useful until they are useful. And when you find out, you are really grateful you've got them.
This. It really is a game changer. I'd always coped fine with single point AF and recomposing, but Eye AF just makes things that much easier. So I think for those 3 things, I'd choose mirrorless over DSLRs any day, now. I was a sceptic, now I'm a convert.