Most medium format cameras have sensor aspect ratios of 4:3 (the Leica S2 at 3:2 is an exception). All 35mm DSLRs from Nikon and Canon use 3:2 aspect ratio sensors.
If you intend to make 16 X 20 prints or similar aspect ratios, the 4:3 aspect ratio fits better (less cropping); a 3:2 camera requires considerably more cropping to fit 16X20. On the other hand, I print on canvas wrap, and so I completely ignore all conventional print sizes.
The choice of formats gets into personal preferences, and here 4:3 could be argued as relatively ideal. Then again, the golden ratio is 1.6:1, so pick your preference. My personal preference would be for a square sensor, along with electronic masks in the viewfinder at the touch of a button, for any preferred aspect ratio. No more camera back and forth for verticals and horizontals, and no wasted lens coverage. Some Nikon cameras offer this today (but no quick button for changing it).
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