There are other sorts of adaptation. One of the more ridiculous chains of adaptation decay is the one followed by The Wonderful Wizard of Oz, which goes:
-The Wonderful Wizard of Oz, book, by L.F.B
--The Wizard of Oz, film, starring Judy Garland
---Wicked, book, by Gregory Maguide
----Wicked, musical, starring Idina Menzel
Each is based on the preceeding, and to some extent on everything before it, but it's mostly a straight chain. And which is better?
Well, frankly, it depends on what I'm looking for. Wicked-the-Musical is great fun, but Wicked-the-novel has a deep political element that didn't make it into the musical, and is explored further in the sequels. Oz-the-movie is a classic, of course, and far nicer than either version of Wicked - in the sense that you come out of Wicked depressed, not cheered, particularly in the case of the book. And Oz-the-novel? Well, it is the start of a massive series, which will take far longer to read than even the four Wicked books - and I like to read.
So it's a case of swings and roundabouts. Same with Jurassic Park, previously mentioned - I love both the film and the book, the film because, y'know, dinosaurs, and the book because it delves far more into the science of those dinosaurs. The book's also a thriller, with the element that the characters - and the reader - don't know what's happening most of the time. The film is far more of an action film, even though the dinosaurs are hardly ever in it. So what mood am I in? That's going to influence your answer.
hS