WS return type, XML string or XmlDocument, which is better?

[Bob]

I'm developing a set of .NET WS for internal use. The server consumer is
also .NET so compatibility is not an issue. I'm weighing on whether my WS
methods should return the data as an XML string or XmlDocument object. To
return as a string, the server ends needs to convert to the XmlDocument
object to a string, and the client end needs to read the string into an
XmlDocument, not pretty, but if the return type is XmlDocument, serializing
and deserializing it take time too. Does anybody know which is better from
a performance perspective?

Thanks
Bob