Integrating IS 61 with SAP SRM 40


I’m currently involved in a project which should integrate WM IS 6.1 with SAP SRM 4.0.
My question is:
what is the best way to integrate IS 6.1 with SAP SRM 4.0?

I guess that the common solution would be to use SAP package and integrate with IDoc or BAPIs. The problem is, I really don’t know what the standard format coming from SAP SRM 4.0 is. Is it IDoc like in R/3 or rather XML? Is it necessary to use the SAP package since there is an XI module built within SRM (in this version), the standard format in XI being XML (more precisely, SOAP messages with enhancements).

Any information concerning this topic will be gratefully welcome.