Follows the manga reasonably faithfully — the most reliable anime entry point.
Diverges significantly from manga plot into original content, compressed due to catching up to ongoing manga publication.
Paces Kaneki's psychological transformation across far more material than either anime season allows.
If you want the more complete, carefully-paced version of the story — especially the protagonist's psychological arc — the manga is generally considered superior to the anime, particularly past season 1.
The anime's production reportedly caught up to the still-ongoing manga's publication pace, requiring an original, compressed continuation rather than direct adaptation.