They are both excellent cameras. The big difference between D750 & D7200 is the D750 has a full frame sensor which is larger than the APS-C sensor in the D7200. Where you will see the difference is in low light situations where the large sensor will capture more light.
Full frame cameras require lenses designed for them. In the Nikon world, full frame lenses are designated FX versus DX for the APS C cameras.
D750 & D7200 are capable of stunning images which can be printed in large sizes, but if money were not an object and I was printing large prints consistently I would go for the D750. Or if I was shooting sports, the D750 would work better as it had a bigger memory buffer if you are doing continuous shooting.
Nikon D750 Autofocus
Auto focus, along with sensor technology, is at the core of professional camera performance. For the way I shoot (often wide open at ƒ1.4), manual focus is not fast enough. Missed auto focus is a missed moment. A camera that nails focus boosts your confidence. Confidence and creativity are intrinsically linked.
I manually select the AF point and employ continuous focus (AF-C). I don’t need the camera to determine the focus point, my composition and eye does that. All the camera needs to do is nail the object I’m focusing on.
After running it through its paces, I’m glad to report that the AF on the D750 is outstanding; my creative freedom of movement was unimpeded. Both center and outer AF points performed admirably. I pushed the camera and expected to see missed focus shots, but nearly every time the D750 surprised me with absolutely spot on focus. It is better than I had hoped, and I had high hopes.
The ceremony – walking down the aisle – shot you see at the top of this post was in very low light. The D750 had no problems whatsoever nailing focus with an outer point (lower middle). Being able to take that kind of shot and not rely on the center point is a huge boost for composition ability.