SQL & PL/SQL

Hi,
We are planning on ERP using SAP.
As we know, that for the customization SAP is using ABAP ( this is what I know for now).

I am looking for advantage and disadvantage from your experiences about writing the SQL statement inside the ABAP versus calling a PL/SQL stored procedure.

I worked previously that using Java vs PL/SQL where the database work, fully handled by PL/SQL Package, and the Java front end only as a control and front end manipulation and validation.
not sure if this a correct approach.

any input, links, blog, etc. appreciated.

thanks