Differences In Movement

While both exercises take place in the vertical pulling movement plane. and they both primarily target the back (specifically the lats) and biceps, the way they do it is slightly different.

Pull ups typically use shoulder adduction, where the elbows come down and back from the sides.

Chin ups on the other hand use shoulder extension, where the elbows come down and back from the front (neutral grip fits in this category as well).

The difference isn’t huge and it doesn’t make one exercise better or worse than the other.

It just means that both exercises train the lats in a slightly different way, and if your goal is to build muscle/get stronger (and avoid overuse injuries), it would probably be a good idea to avoid always neglecting one type of movement in favor of the other.