In Tierra, a cell is first allocated a new block of memory, then writes a copy of itself into this memory, and finally `divides', signalling that the block of memory is now a new organism in its own right.
There is not a great deal of difference between the two mechanisms, but an advantage of the Cosmos method is that it allows an organism to reproduce (i.e. to create a child organism) and to grow (i.e. create a new cell which remains a member of the multicellular organism) using exactly the same technique.
In contrast, Parallel Tierra includes a split instruction which adds an additional CPU to the processor structure of the program. This mechanism is natural for a parallel machine architecture with a shared program space, as used with Parallel Tierra. In Cosmos memory is not shared across cells, so that a multicellular program must actually copy itself from one cell to another in order to run in parallel. With this type of architecture, it seems preferable that the bulk of such copying work should be performed by the cells themselves rather than by the Cosmos operating system (but see the further discussion on this topic in Section 7.2.3).
Additionally, having very similar mechanisms for growth and reproduction of organisms is arguably more analogous to the way that multicellular biological organisms may have evolved.