D7100 1.3x crop in sensor vs Crop in Software?

In crop mode the system will meter only using the area selected.  In a full DX frame image it will meter with all of the light hitting the sensor (light meter).

So in crop mode you will get a more 'accurate' meter to what you are framing it the subject falls into the 1.3x crop area.  If you crop later you will have exposure (metering) based on parts of the image you do not want and are removing.

This will likely only result in small exposure differences (none of course in manual mode), unless direct blackness or sunlight are in the outside edges of the frame.