Does the book add significant new ideas or is it just more explicit about the consequences? I generally love scifi for the ideas much more so than for story and am torn between the book and the short story.
The main thrust of the story (the main sci-fi component) is contained in both. But it is fleshed out better in the full book. However, neither is a story that contains a ton of novel Sci-Fi ideas. For that I would direct you to stories and books by Greg Egan.
I'd recommend picking the story or novel depending on which you can finish in one go. It's gripping and I feel like breaking it up might take away from the experience.
