light novel or manga?

The Light Novel is pretty much always the best option. It's the original source, and the most detailed one, after all. It's very hard to adapt them into a media relaying more on visuals than textual descriptions, which is why a lot (more or less important details/precisions, depending on the story and what the director/Author choose,) gets deleted, especially in the Mangas (which are also often in a monthly schedule).

Be aware that for this work, the Anime already adapted more than the Manga (which is only at around the end of the 1st Season), and it also catched up to the Light Novel (which currently has 9 Volumes, with the 2nd Season adapting until the end of the 8th), tho. If what you want is a continuation, you'll have to wait for a lot of time.

In this case, i'll add a few precisions.

The Author started with the WebNovel. It's an amateur-ish work the Author made himself, and published for free on internet. He was later offered a contract by a professional publishing company, to enhance his story with the helps of professionals, and officially sells it, with the WebNovel serving only as a rough draft. (Therefore, the events are often very different in the Light Novel from how they were in the Web one, keeping only a certain common base.)

So while the Light Novel is the "true" version, the Web one is how the Author originally thought of doing his work.

The Light Novel is licensed by Seven Seas, and you can find some illegal scans if you search enough. The WebNovel was partially translated, but the teams died, so it's hard to find nowadays. The free Japanese version is easily available, and it's still being updated too, tho.