I'm not sure about the rendering issue, but from what I remember about the .ma vs .mb, ma is an ascii file, which you can edit in a text editor if need be, but it is a larger file. mb is in binary, so obviously it would be hard to decipher if something went wrong, though it is a smaller file. As for preferred use, I guess it would depend on what your priorities are (size, safety?). Someone correct me if I'm wrong here