mainly we use the obj methods for internal tables and rdb is used for external tables to extract the data. We can use the obj methods even to access external tables after you do an external mapping of the class and properties.
In terms of funcationality both Obj operations and RDB can achive the requirement almost.But performance wise, it's not recommended to use RDB methosd unless we are using external Database to connect and/or we are writing complex data base queries which are not possible with Obj methods.
Obj- operations generate their own SQL while RDB- operations rely on the query written in Rule-Connect-SQL instances.
Obj methods are meant for transactional ease as they have the option of deferred save, rollback etc whereas RDB methods are performed immediately with auto-commit