@lorenbrothers [MENTION=15302]Blade Canyon[/MENTION]
Happy to respond and I'm going to cover a few things with what was asked and going a bit beyond.
I own a D5100, D7100 x2, D600, D750, and the Df.
The D750 and the Df are close. The Df is better, but if I had to choose one it would be the D750. D750's versatility, ease of use if you're familiar with the Nikon lineup, and speed at which adjustments can be made on the fly make it the hands down winner.
D6xx and Df - Between these two I would choose the Df because its low light and high ISO noise is that much better. The D600, like the D750, is much faster to make changes on the fly. But the low light and ISO is that good. If you are not a low light / high ISO shooter then go with the D600.
D5xxx and D7xxx - These are apples, oranges, and dollars. If you can afford the Df, buy the D750.
So who should buy the Df? Between the lower pixel count (which only matters if you print large) and the by design longer time it takes to make adjustments, this camera is for someone that wants a very specific camera. Maybe for the nostalgia build of how you make adjustments, or in my case, the ultimate low light sensor, but this camera is not a first choice as an all round camera.
lorenbrothers, between the D810, like the D750, there will not be a huge performance jump in low light / high ISO with the Df. However, it does sound like you much prefer the old mechanical versus the modern buttons. This could very much be a match for you. A point to keep in mind, the D810 and Df are worlds apart in operation and may cause frustration jumping between the two.
Blade Canyon, the Df is leaps ahead of the D600, but not worlds apart from the D800. Since you have the D800 I would tell you get the Df if you just want the best low light capability and everything else is just not that important. If that's not what you're in pursuit of then skip it.
I am happy with my Df, but only because I have other highly capable cameras for most everything I do and mostly use the other cameras. However, if the low light performance is all that matters then I grab the Df.
That was long, but I hope this all helps.