Well, the plot of Highschool DxD manga light novel is centered on the supernatural. Issei becomes a devil and gets involved with the devil society led by Rias. There are battles against other devils, as well as conflicts with angels and fallen angels. Each character has their own backstory and powers. For example, Rias has her own peerage which she uses in Rating Games to gain status. Issei's growth as a devil, both in terms of power and character, is a major part of the story. It's also full of comical moments, especially due to Issei's perverted nature which often gets him into trouble.
Issei Hyodo is at the center of the story in Highschool DxD. He is a normal high school boy turned devil. Rias Gremory is a high - ranking devil who recruits Issei. Akeno Himejima is a powerful and alluring character. She has a certain charm and strength. Kiba Yuuto is a character with a backstory full of pain. His skills in combat are notable. Asia Argento is an interesting addition as she was a nun but her path led her to become a devil. Each of these characters plays a crucial role in the development of the story.
The art style is a major difference. In the manga, you get to see the characters and scenes visually depicted in a more immediate way compared to the light novel. The light novel often has more detailed descriptions of the inner thoughts of characters, like Issei's pervy thoughts which are sometimes more in - depth in the written form.
It depends on personal preference. If you like visual art and a quicker consumption of the story, the manga might be better for you. The manga's illustrations can bring the characters to life in an instant.
It's hard to say which one is more popular in an absolute sense. The manga has a wide appeal due to its visual nature. It can attract those who are more into graphic storytelling and quick reads. However, the light novel also has a large following, especially among those who like to delve deep into detailed worlds and complex character development through text. In Japan, both have a significant fan base, and globally, it depends on different regions and the preferences of different types of fans.
The main difference is in the art form. The manga has visual art, so you can directly see the characters' appearances and the action scenes. For example, the battles are more vividly presented in the manga with detailed illustrations. The light novel, on the other hand, relies more on written descriptions. It can often go deeper into the characters' thoughts and the world - building aspects. The pacing might also be different. In the manga, sometimes due to page limitations, some details might be skipped compared to the more comprehensive light novel.