#1. "RE: need buy a camera. D7100 or a D600 ???" In response to In response to 0
Depends on what your shooting needs are. Telling us more about your subject interests and shooting needs would help narrow down the choice.
As good as the DX cameras have become, FX is still better at low light situations and subject isolation through narrow DOF.
DX provides a lighter more compact kit and generally puts more pixels on the subject for a given subject distance and focal length. And given today's PP techniques you can simulate to some extent the narrow DOF which leaves the biggest difference to low light, weight, size and cost.
So for me it would go to the amount of disposable income and shooting interests.