Why can't it be a little of both? You could surmise that the cleansed soul aether gives them physical form. It's what makes up their tangible body. The memory aether is what then gives them their physical appearance and, err, memories.

The very fact that Otis can exist in two different forms simultaneously is proof enough that the Living Memory is making a copy of the memory aether instead of just storing the original wholesale. If it didn't, then Endless!Otis wouldn't exist. If Endless!Otis exists, then Robotis wouldn't.