Functionality difference between Position vs manager hierarchy

Hi,

  i have created following business model for both position & manager hierarchy 

                                              Root bu (ceo position)admin or bu00

                                                |                                       |

                             bu1 (manager1) u2      ===            bu11(manager2)u22

                                    |           share same security role             |

                             bu2 (sales1)u3                                               bu22(sales2)u33

i can't find the uniqueness in them,both kind of work same. 

what is the major functional difference between the position& manager hierarchy?hw it works based on roles,privilege,BU setting?