So I just finished my third re-read of 'Akira', and every time I get to that final volume, the sheer scale of the ending hits differently compared to the film. The manga gives you the full, sprawling conclusion. You see Kei playing a much more active role in the final psychic showdown between Tetsuo and Akira's dormant power. Tetsuo's whole arc gets a proper climax where he finally understands his own nature and then, well, creates his own universe. It's less ambiguous than the anime's famous 'That's... Akira?' line.
The anime's ending, while iconic, feels like a freeze-frame in the middle of the story. It captures the visual poetry of destruction but leaves out the metaphysical resolution. The manga spends a ton of time on the post-cataclysm world and the new psychic children, including the girl Takashi saves, who becomes a major figure. Kaneda and Kei's relationship gets closure, too. The film's climax is about stopping Tetsuo; the manga's is about what comes after, and it's way more hopeful in a strange, cosmic way. My paperback spines are all cracked from those last hundred pages.