EPUB needs an e-reader app and does not print with a fixed layout. FB2 is a clean, open e-book format that many readers support. EPUB is at home in e-books that adapt to any screen size; FB2 is the usual choice for e-books, especially from Russian-language libraries.
Chapters are joined in the book's reading order and its pictures are embedded; an e-book with copy protection (DRM) is refused because its text cannot be read. Headings become sections; a heading that arrives after a section's text has begun becomes a subtitle instead, because FictionBook does not allow a section to mix text and subsections.