Hi Kevin,
As you mentioned SAP currently does not provide PI XSLT mappings for PORDCR1.PORDCR103 Idoc. So, if u want to leverage SAP's std. mapping, its better to use PORDCR102 idoc.
But, if there's some strong functional need that only PORDCR103 can meet, then you will need to ask your PI team to reuse the SAP provided XSLT mapping and add custom Idoc field to XML field mapping for the additional fields in PORDCR103.
Regards,
Bharath