Which media follows the original novels more? The Anime o manga?

I have been some what of a Durarara!! fan for awhile. I have read all of the manga published so far and have watched some of the anime. I read the manga first, but that was before I knew that the Anime was created before the manga. I usually read/watch what came out first.

Both the Anime and manga have big differences (even between the bvery first episode and the entire first volume). So, I was wondering if one was più like Ryohgo Narita's original novels, and if so, which one.

If i could have an answer from a fan who has read the novels and manga as well as watched the anime, it would be greatly appretiated :D

(Random picture of my preferito manga fan ever!)