Hello Mala1,
XMLLIB--->VALIDATE(xml, schema_url) is a functionality introduced in WTX. In earlier versions of DSTX, we used to parse at the input type because such a functionality did not exist. I feel both approaches are meant depending on your requirements and built-in system functionality.
My personal opinion is that it might be convenient to use XMLLIB--->VALIDATE. It is because error handling might be simplified this way.
Performance-wise, parsing at the input is fair enaough; although I do not have any data on XMLLIB---->VALIDATE.
I hope this helps!
Regards,
Mondweep.