Have ’em both. Use ’em both. Love ’em both.
But, they are VERY different cameras, for very different purposes. Here are some of my observations about the differences and similarities between the two cameras, and also some of what I have found to be weaknesses in both. Keep in mind that the way you use your camera, and what you shoot, will mean that you may well not agree with my comments. Using a camera to make photographs is a very personal experience and one size does not fit all.
Handling
This one goes to the Nikon. Hands down. It’s a big camera body, and all the controls are very conveniently located. After using the D-850 for a couple of years, I have developed the muscle memory to find every control that I need while shooting, without any thought. The controls are intuitive and easy to find. For example, changing the ISO on the Olympus (at least using the default setup on the camera) requires me to flip the lever on the back of the camera, then use a dial. On the Nikon, there is a dedicated button just behind the shutter to adjust the ISO. Hold the button while rotating the rear dial – easy.
Olympus OM-D EM-1 Mk II with Zuiko 40-150mm f/2.8 lens.
Image Quality
This one is tough. First, I have to say, I don’t have the skill, knowledge, or facilities, to actually “test” cameras or lenses. I can only judge by what I see, and that is clearly subjective.
Olympus v. Nikon – there’s no clear winner here. I have taken photos with both cameras that are completely usable in every respect. What I have found, however, is that the quality of images take with the Olympus EM-1 camera with Olympus lenses (Zuiko) is more consistent than images created on the Nikon D850 with a variety of lenses. I use lenses by Nikon, Tamaron, and Sigma on the Nikon. Generally I can’t tell the difference between Nikon and Tamaron. Both are spectacularly good. Some of the Sigma lenses are every bit as good, as well, but others, not quite so much.
Overall, I conclude that the image quality is more influenced by the lenses than the camera bodies. Both systems are capable of producing extremely good images.
The original Olympus OM-D EM-1 with Zuiko 7-14mm f/2.8 lens
(L) Original Olympus EM-1 (with 7-14mm f/2.8 Zuiko lens) and (R) Olympus OM-D EM-1 Mk II (with Zuiko 14-40 mm f/2.8 lens)
(L to R) Olympus OM-D EM 1, Olympus OM-D EM-1 Mk II, and Nikon D850 with Sigma Art 24-70mm f/2.8