I had been meaning to read the book for, oh, the last thirteen years and never did, but I sat down and watched this last night. Honestly, I think I would have appreciated the book more, but was very impressed none-the-less.
I think the underlying "message" of the story is probably a little bit easier to convey in book format. Specifically, the ending and the presentation of the more plausible story of the journey. With the movie, I think it's pretty easy for audiences to brush it off as a plot twist rather than anything more. Now, I haven't read the book, but from what I can gather from just the movie, the entire dilemma and underlying message of which story you choose to be better is actually the focal point of the story - two stories are presented, one is plausible and believable, but horrific. The other is much much harder to believe, but one in which you want to believe. Since the story itself has no bearing on the matters of importance - such as why or how the ship sank, many would ultimately find it more rewarding to choose the unbelievable story - but that takes faith, "And so it goes with God".
I think many watching would just walk away without all the subtext and just see it as a kind of M. Night Shyamalan-esque twist, which is a shame. It's not a fault of the film itself, but probably more of just a side effect of how we tend to take movies in differently than books.
It reminded me a bit of Atonement. The story you watch for most of the running time turns out to be a fantasy, and the real, brutal truth is hidden for reasons very personal to the storyteller. It's impossible to dismiss. It becomes all too obvious. The story as presented is too fanciful, too ethereal to be objective fact. However, the brief confession under pressure is not. It changes the mood from wonder to shock, then sadness.