Distortion is primarily a property of the lens, not the sensor. But with the same lens, a full-frame camera usually shows more visible distortion because it uses more of the lens’s image circle, including the outer areas where distortion is typically strongest.

An APS-C sensor crops off those outer edges, so the image often looks less distorted. If you crop the full-frame image to the same field of view as APS-C, the distortion will be essentially the same.

So the tradeoff is:

• full frame + same lens = wider view, but more of the lens’s edge distortion is visible
• APS-C + same lens = narrower view, with less visible edge distortion

For Milky Way work, don’t assume a shorter focal length always means less distortion. Different ultra-wide lens designs can distort very differently, and some have unusual mustache/bulge distortion patterns. Field of view and distortion are related but not the same thing.

If your goal is the widest possible sky view with low distortion, compare specific lenses rather than sensor formats alone.