There has been lots of debate over the years about the best approach to managing marked-up content, and it is safe to say that there is not a single answer. This means that you need to understand what the differences are between them – and it won’t be easy for those of you new to the unstructured data world – this is much trickier than the relational data world. In spite of the huge benefits of the major DB players providing serious XML support, the wide variety of content application requirements will ensure a long-term need for quite a few specialty vendors, whether they are targeting vertical applications or horizontal components. The good news is that even with many different XML application schemas, it continues to get easier to integrate all kinds of XML data.
Bill points to an article in eWeek here. Also see Dave Kellog’s comments and links to other articles.