The book had a far better ending. The ending on the movie was the only thing I didn't like.And the movie takes the safe approach and tells the story from McMurphy's point of view, as well as cutting out significant portions of the horrors of being in a mental institution.
Not sure why they decided to go full on tragedy at the end of the movie. I mean the chief escaped but over all McMurphy and Billy both died in vain. At least in the book, their deaths resulted in giving most of the acutes greater purpose in life.