First person novels are often more about self - discovery and the internal journey of the narrator. It can create a sense of closeness between the reader and the protagonist. Third person novels can vary greatly. If it's third person omniscient, it can show different sides of a story, different characters' motives at the same time. For instance, in a mystery novel, the third person narrator can show the detective's thoughts and also the criminal's actions without being restricted to one point of view. Also, in third person, the description of the setting and other characters can be more detailed and detached compared to first person.
The main difference is the point of view. First person is very subjective as it's one character's view. The reader experiences everything through that one person's eyes. Third person can be more objective. It can be omniscient, where the narrator knows everything about all characters, or limited, focusing on a few characters. First person might be more engaging on an emotional level as it's so personal, while third person can be better for complex plots with many characters.