I'm ignoring the spec as I haven't seen Life of Fred! It's clear though that one difference is that Murderous Maths isn't really teaching as such; it is deliberately recreational maths, delighting in not being aligned with anyone's syllabus. (One of the books awards Perfectly Useless Fact badges to some of the things it points out.) Of course, there's no such thing as maths that connects with nothing else, so it will support syllabuses, but only accidentally. Life of Fred, from the sound of it, takes a much more path-like approach. I'd say you can't really go wrong with MM, unless in fact your DS is past it and better suited to adult recreational maths such as the Martin Gardner books.
