Sure is! Solo Leveling is a well-known manhwa that has captured the attention of many readers. It features exciting action and interesting character development.
Another theme could be the clash of different universes. Amazo is from the DC universe and Saitama is from One Punch Man. Fanfics might play around with how these two different universes' rules and powers interact when these two characters meet. Also, there could be themes related to respect and rivalry. Saitama is often very nonchalant about his power, while Amazo is a complex android with various abilities. How they view each other in the fanfic can create interesting storylines.
It's hard to say which one is more popular as it depends on different factors. In some regions, the manga might be more popular because it appeals to those who like visual stories. The action - packed panels and the cool character designs can draw in a large audience. For example, in Japan, the manga format is very popular in general, so the solo leveling manga has a good chance of being well - received there.
The main difference is in the way the story is presented. The manga has visual art that can enhance the action scenes and character designs. For example, the epic battles in the manga can look really cool with detailed drawings. The light novel, on the other hand, usually goes more in - depth with the descriptions of the world - building and the characters' inner thoughts. It might have more detailed explanations of how the leveling system works in the story.