D600 with D7100 as Back up

I am not a believer in the "extra reach" idea that people discuss about the dx format. Every lens draws one image size. A dx captures less of the picture.  At the same mp sensor, a dx may give you slightly smoother transitions, which I see people calling pixel density.  Look at some examples before signing up for that idea.

That said, the perfect backup for a D600 is...a D600.

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Currently shooting aD300s, a D200, D70 and N50. Have a 'F.'
Also shooting with Konica 35mm SLRS (T3 and FT-1) with numerous Hexanon Lenses. Printer: Canon i9900.