What is PLY?

The Stanford format, built for scanned data. Its distinguishing feature is per-vertex attributes: colour, normals, confidence values, whatever the scanner recorded. Research and photogrammetry tools default to it for exactly that reason.

What is OBJ?

Wavefront OBJ is plain text, thirty-five years old, and readable by every 3D application in existence. Vertices, faces, texture coordinates and named groups, with materials kept in a separate .mtl file alongside - which is the part that most often goes missing when a model is shared.