The web novel often has a more raw and unedited feel. It might have some rough edges in terms of writing style. The light novel, on the other hand, usually has better presentation. For example, it may have more detailed illustrations. Also, the light novel might be more refined in terms of grammar and language use as it goes through more editorial processes.
Well, in the Tensei Slime web novel, the story might progress at a faster pace. There could be more frequent updates which sometimes lead to less polished chapters. In contrast, the light novel version may have a more consistent quality. The light novel may also expand on certain plot points or character development more elaborately compared to the web novel. It could be because the publishers want to make it more appealing to a wider audience.
One major difference is the availability and access. The web novel can be accessed online for free in some cases, although it might be in a less organized format. The light novel, however, is a physical or digital product that you need to purchase. In terms of content, the web novel may contain some ideas or storylines that were later revised or removed in the light novel. This could be due to various reasons like making the story more cohesive or adhering to certain market expectations.