One possible difference could be in the level of detail. The novel might go more in - depth with character thoughts and emotions. For example, in chapter 71 of the webtoon, a fight scene might be shown more briefly compared to the novel, where it could be described with more elaborate language.
They fill in the gaps. There are many parts in the main story that left us wondering about certain characters or events. The side stories answer those questions, making the overall narrative more complete.
It depends on your reading level and familiarity with the genre. If you're used to reading light novels, it may not be difficult. The language is usually accessible and the story - telling is engaging.
For some readers, it's not difficult at all. The plot of Solo Leveling is engaging and the writing style is straightforward. However, if you're new to web novels or not into the fantasy and action genre, it might take some time to get used to the pacing and the world - building concepts.