thanks for the interesting comments so far. Of course it's not the idea that the Df should replace the newer mirrorless camera -- obviously the Z mount has excellent (though the top tier ones are completely outside my budget) new lenses and, although I quite like the shutter click of the Df, there are many occasions when the far more subtle one of the Z6 or complete silence is desirable. I've never had an issue with dust on mirrorless cameras whereas in the DSLR's, it always seems to creep in eventually. Magnification is easier with an EVF to check results.
On the other hand, I've never actually liked EVF's despite their practical advantages - they still have some way to go. They consume far more battery. My Df can AF my beloved 80-200 f2.8D more accurately than MF with the Z6 and, like my other F lenses which still outnumber the Z variety, can be used without the ugly adaptor. Being able to preset aperture and/or shutter speed and ISO without turning on the camera can be a huge boon.
Purely with the sensor, they are different enough to warrant both and initial impression are somewhat in favour of the Df. Of course, other than backlighting which gives a slight improvement to higher ISO, the Z6 sensor is not really different from the D600 of a decade ago. There's no evidence that using current technology, sensors will significantly develop further.
