Someone asked me which one I would choose, the Df or the D610?
I get asked these type of questions often. I've discovered that people spend a lot of time and energy researching cameras, often a lot more time than learning how to create great photographs. But cameras are not what's important in photography.
"The single most important component of a camera is the twelve inches behind it." --Ansel AdamsWith all of that said, here is my opinion: I'd buy the Nikon Df over the Nikon D610. Why? Because the Nikon Df would be more fun to use. Because the Nikon Df would give a more organic, film-like experience.
But what I think doesn't matter. Both cameras are fantastic. The D610 is less expensive by $700, so if money is an issue, go with the D610. Image quality will be nearly identical between the two cameras (although the D610 will produce larger files). Go with your gut and pick the one that interests you the most. You cannot make a bad choice here, because you are choosing between great and great.