The manga of 'A Certain Magical Index' and its light novel counterpart differ in how they present the story. The light novel can be more complex in terms of plot and character development. It allows the author to use a large amount of text to explore the philosophical concepts underlying the story, such as the nature of power and morality. The manga, being a visual medium, simplifies some of these aspects to focus on the more immediate action and the relationships between characters. It uses panels and art styles to create a different kind of atmosphere. However, both have their own charm and contribute to the overall popularity of the 'A Certain Magical Index' franchise.
Well, in the light novel of 'A Certain Magical Index', the narrative is more in - depth. It has the luxury of using a lot of text to describe the inner thoughts of characters like Touma. In the manga, the expressions and body language of the characters play a big role in conveying their emotions. Also, the light novel might have more side stories and extra details that don't always make it into the manga adaptation. So, if you want a more comprehensive understanding of the story, the light novel is a great choice, but if you prefer visual impact, the manga is better.