Since both the manga and the novel came out at almost the same time, and they're practically identical in every detail, I have to assume that it was only a publishing decision to release the manga (in Japan) several months before the prose. Not surprising, really, since most major fiction in Japan is manga. 
 Being an obsessive anglophile and a fan of the tale in all its incarnations, I wanted to nitpick any plot differences between the movie, manga, and novel. I didn't mind in the slightest that I was rereading the manga. In some ways, the novel was better if only because my mind is allowed to fly free with additional sensory input that doesn't quite exist in the novel. You know. Imagination. 
 It's a great military novel and the ending is great. I kinda wish that we could have skipped the hollywood ending in the movie, but damn.. you know, it's hollywood. 
 I'm going to recommend both this and the manga equally. There's no practical difference except in the amount of drawing. 
 It's one of the best nightmare versions of a daydream I've come across.