CO_item object is very powerful object and capabale of archiving data from most of co tables and for almost all object types. By object types I mean WBS level/ Internal order level/Cost center level/etc.
Depedning on your landscape you need to choose the object in varinat configuartion.
However you will not be able to get lot of control in terms of configuation for CO_item
CO_order and PS_project also archives data from above tables but you get lot of data control in these object.
If you have Result analysis implemented on your landscape you need to be xtra cautions when deleting data from CO-item as input for RA is impacted by CO_item.
CO--item is follwed by Co-total for co data deletion.
Hope this helps.