Having bounced off the book a couple times in high school, I finally picked up the film on a whim last year. It's a powerhouse, no doubt, but it felt... contained. It's Jack Nicholson's movie. Picking up Kesey's novel afterwards was like opening a window in a stuffy room. The prose is this wild, swirling thing, told through Chief Bromden's fractured perspective. You get the texture of the asylum itself—the fog, the sounds, the history—in a way the camera just can't capture. The film is a brilliant, focused character study of McMurphy versus the system. The book is the system itself, breathing down your neck.
If you're after a tight, iconic performance and a clear narrative arc, start with the film. It's a masterpiece of adaptation that understands its own medium. But if you want to get lost in the psyche of the place, to feel the institutional oppression in your bones from a first-person account that's both heartbreaking and strangely beautiful, the book is the only way to get that. Personally, I'm glad I saw the film first; it gave me a solid framework, and then the novel blew that framework wide open with all its messy, brilliant depth.