1946 is definitely better. It's true noir (in b&w), tough and scary, with a neat twisty mystery, an excellent cast, great music and direction.
Don't get me wrong: 1964 is very good, and some of the plot changes are really interesting. But Lee Marvin is most of the show (though the cast here is good too), and the movie suffers from the cheap production values Universal put into it because it was intended to be a made-for-TV movie. (Obvious and familiar back-lot scenes, phony backgrounds, bland music and cinematography, even crummy credits -- a film clearly shot in a hurry, characteristic of TV movies, which are usually quickly and shoddily made.) It would have been much sharper and of better technical quality if it had started out to be a theatrical film in the first place.
I've never read the Hemingway short story, but from what I've read about it, 1946 sounds closer. But his story apparently ended with the killing; as I understand it, the plot used in both films (investigating why the victim didn't run) is absent from the original story.