D7100 1.3x crop in sensor vs Crop in Software?

Not necessarily.

Example: using the full frame, you position your subject 1/3 in from the right edge of the frame. You may be outside the sharpest part of your lens.  So, if I crop that in post, I may not be using the sharpest pixels.

Now, frame the same subject a 1/3 in from the right side of the 15 MP crop area, and you're now closer to the center of the lens. Make sense?

The 1.3x crop mode uses the center 15 MP (approx.) of the full 24 MP frame.