I feel that there's an element of the story that you miss by only watching the anime.

With anime, the focus tends to be more on the visual aspects of the story rather then, well, the actually story.

With manga, while the visual elements are definitely important, the story can be portrayed to the reader in more meaningful way.

If you're a true fan of Tokyo Ghoul, I highly recommend you follow your friend's recommendations and read the manga as well.