what are the advantages and disadbvantages of hierarchies over nav attributes and vise versa.
Hierarchies you use basically if you wish to show your data in a tree structure, however navigational attributes don't give this facility to show your data in such manner.
Navigational attributes are basically used to haver add on information for infoobjects, which is quite handy while drill-down in queries.
Hope it is somewhat useful to you!!!!