Hi Christophe,
No, we don't compare to other agile methods in the "Why Agile?" chapter. It's more of a brief rationale for agile development. We talk about the danger of thinking of agile as a silver bullet and how, despite its current popularity, agile development is only worth trying if it can improve your ability to deliver successful software.
Then we spend some time talking about the flaws of the traditional definition of success ("on time, on budget, as specified") and present an alternative definition of success instead ("organizational success, technical success, and personal success"). We wrap up by talking about how agile development might help the reader be more successful in those three areas.
In the second chapter ("How to Be Agile"), we explain why we chose XP, but again we don't compare with other methods. Here's our explanation:
The Road to Mastery
The core thesis of this book is that mastering the art of agile development requires real-world experience using a specific, well-defined agile method. I've chosen Extreme Programming for this purpose. It has several advantages:
Of all the agile methods I know, XP is the most complete. It places a strong emphasis on technical practices in addition to the more common teamwork and structural practices. XP has undergone intense scrutiny. There are thousands of pages of explanations, experience reports, and critiques out there. Its capabilities and limitations are very well understood. I have a lot of experience with XP, which allows me to share insights and practical tips that will help you apply XP more easily.