What is OFF?
OFF (Object File Format) is about as small as a mesh format gets - a header, a count of vertices and faces, then the numbers. It turns up mostly in academic geometry work and mesh-processing datasets, where being trivial to parse matters more than being widely supported.
What is PLY?
PLY, the Stanford triangle format, came out of 3D scanning, and it shows: it carries per-vertex extras like colour and confidence that most mesh formats have no room for. It exists as ASCII and binary; FileSorcery reads the ASCII form.