Now that the Nikon Z is announced and I know what it is, and what it isn't... I'm thinking about buying a new Nikon DSLR. I used to have a d810 but sold it before the d850 was announced and dropped the value too much. My only dislike with the d810 was how big and heavy it was.
On eBay I see some new Nikon Df bodies for ~2,100 -2,300 USD$, and some used Nikon D4s bodies for ~2,500 USD$. I have a 50mm f1.4 ziess milvus (manual focus lens) and split prism focusing screen.
I've herd the D610 is better than the Df bar the low light. I like the Df size more than the D4s, but have never used a dslr with a grip. I like the fact that the D4s has expeed 4 (df expeed 3). I like that the Df has an AE-L/AF-L button (I don't know if the D4s has another solution). I like that the D4s has better auto focus, but right now I have one manual focus lense and a split prism focusing screen. I bet the D4s would encourage me to buy other lenses and offer a lot more features however I mostly shoot in the woods (plants animals and fungi), on farms, kids, pets... slow moving subjects I easily nail the focus on first shot, I'm very used to and good with a manual focus lens.
My two biggest considerations are low light and size.
Talk me into either camera