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 popularity of the Highschool DxD manga and light novel varies among different groups. Among younger fans who are more used to visual media, the manga might be more popular as it gives an instant visual representation of the exciting story. But the light novel has a strong presence among hardcore fans who appreciate the in - depth storytelling, detailed world - building, and the original vision of the author. They like to analyze every word and description in the light novel, which the manga might not be able to convey as thoroughly.
I think it really depends on the market and the type of audience. In some parts of the world, the manga is more popular because it's more accessible to a wider range of people, including those who might not be as comfortable with reading long texts. But in certain fan communities, especially those that focus on in - depth literary analysis of the series, the light novel is highly regarded. Also, the availability of translations and marketing efforts in different regions can impact which one is more popular in that area.