I've read the argument that the OD is not as strong, but I have to wonder. Doug Dutra's article on SlantSix.org says otherwise. He clearly says that they are both the same strength. His not an opinion I would consider fly by night, either.
From what I have read I gather that the aluminum case o/d is weak and the cast iron one is much better strength wise but not as tough as a standard 4 speed
