Generally all queries should be through the use of Obj methods. If you use RDB methods, you will not be able to access BLOBs. However, RDB methods are necessary if you want to execute stored procedures.
DB methods would be considered in the below factors
In comparision with efficiency, RDB methods gives good efficiency but conditions apply* that also depends on the business scenario. Obj-methods communicates with classes followed by it will interact with Databas Table. WhereAs, RDB methods contains SQL Queries that directly communicates with the database and performs the query.