Amplifying Richard's remarks, the two cameras are not very comparable. I've hardly shot with my D750, but in terms of controls it's a near clone of the D7xxx cameras I've used for years. Those cameras have a very decent control system, though now that I've been exposed to the pro controls I find these ones a little bit lacking. The D750 is a much better camera for typical digital shooting with autofocus lenses. It offers a lot of other nice features.
By comparison, using the Df is a mixture of joy and frustration. Joy in that it's easy to focus old manual-focus Nikkors using the ground glass, making my collection once more readily accessible for handheld shooting. It reminds me a little of my film days using an FE, except that back then I had a split screen to focus with. The controls are a mixture of F3 and D750. While the shutter speed dial is nice enough, the ISO and exposure compensation dials look nice but are not as easy to use. The mode and front command dial are awkward and frustrating. The front grip is nowhere near as nice as with other Nikon DSLRs. I end up tolerating this camera's shortcomings so I can gain its access to manual focus lenses including the pre-AI ones. It's not a camera for fast shooting or adjusting settings on the fly.
So IMO it comes down to what kind of lenses do you have/want and how do you want to shoot?
-Slow, contemplative handheld shooting, sometimes with manual focus lenses? Df for focusing on the ground glass and access to pre-AI lenses. The quirks will be tolerable because you won't be in a
hurry.
-Shooting on a tripod with magnified live view focusing? D750 for the resolution, features, and flexibility. Better yet, get a D810 or D850.
-Shooting on the fly using autofocus (action or street)? D750 for better AF and ability to quickly change settings while keeping your eye to the viewfinder.