I've read both, more than once. I must admit I prefer the short story because it is more concentrated in essence, and maintains the mood and the arc much better for the tragic effect, making an intensely powerful experience that I still cannot read (after more times than I can count) without finding a huge lump in my throat and tearing up. The novel I initially didn't care that much for (being used to the story), because I felt it was a dilution of that effect. They should be judged differently, though, as the novel wanders from issue to issue along Charlie's journey, much as life itself; when seen as his odyssey, with all the various changes rung fully developed, it becomes a glowing experience in its own right. However, I still feel that the short story has the advantage of single concentrated effect that hits with the force of a piledriver. The novel is more contemplative, and therefore the emotional impact is somewhat tempered by the intellectualized experience.