The Second Edition is a separate book, as it's a rewrite. The first and second editions are being treated as entirely separate. The second edition is what I consider to be a more definitive version of the story: I've added scenes, removed others, rewritten pretty much every chapter.
The most obviously example is that Kelly's journal is now written in the form of a memoir, so is a lot more expansive (it's also more consistent with books 2 and 3). There's also a new chapter called "What Natalia Saw", which reveals informations about her past. Dodds also talks more about Poppy Castro and Stefan Pitt...
and that scene where Estelle has to fetch Kelly from the dorm, so that she doesn't miss the patrol? Yeah, I always found that quite grating, so that's gone entirely..!
Compare the sample chapters of the first and second edition and you'll probably find yourself leaning towards the second
