I’ve written a small fbx exporter using FBX SDK, the only difference between both files is just a simple tag that sets the fileformat
lExporter->Initialize(pFilename, pFileFormat, lSdkManager->GetIOSettings();
pFileFormat could be initialized to several modes like binary, ascii, 6.0 binary, 6.0 ascii, dxf autocad etc…
Obviously binary is much better than ascii since it gets exported faster, takes less space and gets readed faster too, but some objects fail to be loaded, notice that ascii one never fails neither 6.0 binary but this last one is a no sense since it takes almost the same space as ascii too.
I’ve attached an example object in ascii/binary so it is easily preproducible: both models inside a zip file