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 OFF?
Object File Format, and about as plain as a mesh can get: a count of vertices and faces, then the vertices and faces. It comes out of academic geometry work, where nobody wanted to parse anything more complicated to test an algorithm.