The main advantage of DSLR cameras over mirrorless cameras used to be that what you saw in the viewfinder was (and still is) exactly what your lens saw, and therefore what was going to be on the picture. Not necessarily exactly what your photo would be, as effects of lighting and depth of field were not visible (these are amongst others affected by the shutter opening you chose, some cameras have a button to apply that to the viewfinder). Mirrorless cameras used to show an imperfect view of that, and usually with some delay, and pixelated. Those cameras have much improved over the past years, to the point that the quality of the viewfinders are comparable.
When buying a camera today, it’s really more about your preference than about which one is better.
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