I don't own "Professional XML" so I can't really comment about the comparison between the two books. Looking at the table of contents on Amazon, the contents look similar, however some of it may be a little out of date. I notice that it refers to IE5 and that it was published in 2000. Maybe there's a more recent edition that I'm not aware of.
Thanks for the link to the IBM exam. Based on the information contained in the page, the book addresses all areas listed in sections 1 and 2. The book does not address the SAX 2 API from section 3 in any detail. It also does not cover XSL-FO (XSL Formatting Objects) mentioned in section 4. Given the contents in section 5, you'd need to have some real world experience in developing XML applications. Although books can point out some of these areas, they won't give you the hands on work that you'll need.