I have argued for slugs
instead of chapter numbers in the URL's. There are many links to the book on the internet (here, Stack Overflow, etc.), and if we ever re-number the chapters (which we have done in the past), all the old links break.
Also, when a section title ends with "(experimental)", it would be nice to keep that out of the URL, so when we finally remove the "(experimental)", it won't break existing links to the section (not to mention the fact that the parentheses present problems when pasting the URL into posts on Stack Overflow and other places).
Anthony