I believe all of the three versions produced by Towers (and also the old Rene Clair one) use the alternate, more uplifting ending unlike that in the book. In all fairness though, it was Agatha Christie herself that came up with this new ending when she adapted the story for the stage. It's a less grim conclusion and hence more appealing to a "mass audience" so I guess that's why most of the filmmakers who adapted the story for the screen have opted for this ending. There is, however, a Russian version, DESYAT NEGRITYAT (1987), that retains the original conclusion from the book.