Is it difficult to develop something on Oracle XML DB? The functions are normal maintenance operations such as add, delete... Moreover, how about the SECURITY? How to set the object access right in Oracle XML DB?
If they both provide access through JDBC the effort should be pretty similar.
Personally I think XML is WAY overused, mainly due to upper management in many companies dictating it be used everywhere without any knowledge of what it is or what it's good for.
While it might make sense sometimes to store XML in a database or to store data in XML format it makes no sense doing so for all data under all conditions.