I’ve only watched the anime so I cannot compare to the light novel. Volumes 1-2 are basically a copy of the anime, but volumes 3-4 diverge by a great deal. The interactions and scenes are completely different with a very unexpected ending that gives a new meaning to “Occultic;Nine”.
Story: 10/10. The pace and direction was excellent. Unlike the anime, the manga has a story that flows very well as a complete package. The ending is also surprising and has a strong effect on the manga as a whole. There’s a lot more meaningful and confrontational moments, especially with the manga’s villains.
Art: 8/10. Very good but not particularly outstanding. There are certain panels that are drawn to great effect, but there are plenty of simple panels with white background.
Characters: 8/10. Again, the characters are very good but not particularly outstanding. Unlike sister series like Steins;Gate and Robotics;Notes, I didn’t feel there were any lasting connections between the characters. However, there wasn’t a character that was written badly or anything of the sort.
Overall: 9/10. This is definitely worth your time.