The Web Novel is usually the original draft of the story created by the author with no editor. The Light Novel is the actual story and is usually more polished and refined. The WN and LN of a story can be pretty similar with very few differences e.g. Mushoku Tensei, or they can be almost completely different stories with the same characters and setting e.g. Tensura. I would always advise you to read the LN since it has been edited and is usually just better to read than the WN. It can also contain content that didn’t appear in the WN, such as short stories or even other important events that deviate from the WN. The LN usually doesn’t cut parts of the WN, unless the author intends to change it. If anything, the LN should have more content than the WN, but that isn’t always the case.