There is nothing scientific here. I am quite sure there must be tons of comparisons in the Net with articles full of useful information. I base my comments on personal shooting experience. I have not made enlargements beyond 12x18 inches for comparison simply because I seldom go larger than that. I know that others using mirrorless bodies have gone beyond 12x18 and they have been very pleased with the results.

I use Olympus cameras and lenses. I used my D610 with the 70-210 f4-5.6 zoom lens while my Olympus body was the EM-10 with the Zuiko 12-40 f2.8 Pro. To begin with those are two different lenses but both quality lenses with very good resolution. While the D610 uses a sensor similar to a 35mm film camera the Olympus has a smaller sensor. Other mirrorless cameras like the Sony full frame have a similar sensor to the D610. We all know that there are different mirrorless bodies with different size sensors. I am making my comments based on what I use so the experience of others could be different using different gear.

In the first place, my mirrorless camera is the EM-10 Mk. II, much smaller and lighter than the Nikon D610. Technologically speaking the mirrorless beats the D610, it is smaller, lighter and capable. At the end of the day if using the D610 with its lenses I will be tired while the mirrorless will not send me to bed to rest. Both cameras have nice interchangeable lenses and I mean quality lenses. I have traveled with both cameras and I prefer to travel with my Olympus.

Exposure is easier with the Olympus because changes made to the exposure can be seen in the monitor. With the D610 even in live view I have to go to the histogram. The result is that with the Olympus I spend less time checking the exposure. Shooting action or wildlife you better leave your Olympus at home and I have no experience with the EM-1 Mk II and its AF system which is supposed to be pretty good for those subjects.

Comparing both zooms is a gigantic task. They are totally different and for different purposes. I would say both are capable of fine images with each camera. The Olympus colors to my taste are better than those made with the Nikon body but those colors can be modified with software. I used the files in the computer and cropped to 50% and both are very capable of at least 16x20 inch enlargements. Keep in mind that the Olympus body has 17 megapixels and the D610 has 24 so theoretically we should expect more resolution with 24 megapixels and a larger sensor. I did not see that difference and let me repeat it, this is not a scientific test. I am basing my comments on what I saw.

Which one to use? I would say both are excellent cameras and if wildlife or sports are your subjects the Olympus should stay home. For portraits and general photography both do very well using good optics.
The technology built in the Olympus is superior to that in the Nikon body by a large margin. Olympus has lots of parameters hidden in the menus and that could be complicated till you learn the camera. Using the D610 should not be difficult to those that have used Nikon bodies in the past. Because of the lack of a mirror the Olympus seems to work quieter.

I posted images with the Olympus last week so I am repeating here similar images made under the same lighting conditions but this time with the D610 and 70-210 f4-5.6. Quality wise as I said I saw no big differences when I opened the files in the computer and went to 50% crop. I do not believe I will see a significant difference between both files with my usual enlargements to 12x18 inches and I seldom go larger. Others as I also said have gone much larger with excellent results. I will print some of the files from both cameras to 12x18 inches in a professional lab but from what I have seen in the past my eyes have not been able to tell the difference.

Except for sports and wildlife you can be confident using a mirrorless with good optics and since I do not have experience using Sony or Fuji I do not really know how those cameras do with sports and wildlife. Perhaps others using those cameras can add to what I have said here.