XML vs Relational: Fabian Pascal puts it best in this article. The man says what he means.

I really like that Oracle XDB. If you are going to use an XML DB it looks like their shredding technique comes as close as possible to automating the decomposition of documents onto tables. But only top level documents get inserted into tables -- and those tables are of "XMLType". XDB describes nested elements by new types --domains-- it constructs: good. But I cannot see the storage layout for instances of those types: not good.
Finally, I was hoping XDB would permit me to annotate the XML schemas to fully map instanced documents onto tables of my own design, or onto legacy tables -- but that is not the case. I could write triggers of course to map document constituents onto the tables. But I already could do that!