Trying to decide about replacing a camera body with a newer body is very similar to deciding between a Porsche or a Ferrari. Both have marvellous specs and both are going to get the job done in amazing fashion. Both are better than what you are driving right now.
Still it comes down to what do I need and what do I want? I think I had decided on the Nikon D610, I am already heavily invested in Nikon gear, then comes along the Nikon D750. Both brilliant cameras with amazing specs. The D610, like its predecessor the D600 have over 24 mega pixels. More than enough to get the job done. Pretty much any job. Commercial billboards, giant sized prints, anything. Far better than my ancient D70s’s. Their measly 6.1 mega pixels can get it done for most jobs but 13 x 19” is pretty much the limit for printing size.
The D750 similarly also has more than enough pixels to get the job done. The Nikon Df is the type of camera I have wished camera designers would create more of. A more intuitive simpler camera similar in operation to my old Nikon FM2n. A workhorse of a camera that I knew backwards and forwards. However I do wonder why Nikon didn’t give it a bigger sensor to compete on more even terms with the big boys like the D810 or the D4s?
So in this case I am going to defer to the logic I give others when asked the unavoidable question most photographers get: “What camera should I buy?” Pick a price point and invest in the “best” camera you can afford. To that end I think the D610 is still the camera of choice. That doesn’t mean I won’t still covet the others, but I shall still remain comfortable in my choice.