>for all kinds of photography, astronomy, landscape, >panoramics, travel, flat field copying, etc. My need for the >accurate focus is shooting ballroom dancers under less than >ideal lighting, fast motion. >
Hate to say this, but your D600 was the ideal camera - there is going to be no comparison with a DX sensor based camera above ISO 1600, though the D7100 looks pretty nice and is the most like the D600 from a control and capability perspective.
"Extraordinary claims require extraordinary proof " - Carl Sagan