I might try DeltaXML + an XSLT transform to get the specific result that you want. One problem with using plain-text compare tools with XML is that the physical form of an XML document (its indenting, etc.) can change even if the content itself doesn't, which could lead to a lot of false positives. DeltaXML is a very good tool for comparing document-based XML.
Also, depending on the editor your users use, it may have XML differencing built-in. For example, Arbortext Editor has a good tool that shows the text (not the tags) with changes highlighted / struck-through.
