Light novels vs manga

Sort of depends.

LN's usually have more content since the writers are only limited by their imagination more than a budget. That means LN's usually don't have rushed plotlines compared to anime, as seen in some of the adaptations that the fans thought were terrible.

But with that advantage, you're also giving up voice acting, music, and moving visuals.

You should probably also know that in a lot of cases the manga adaptations of LNs are not done by the original authors, small changes are to be expected to fit the medium just like with changes to anime adaptations (some things are just better done in different ways when adapted to a different medium). Often manga adaptations of LNs don't even end up reaching far in the story, sometimes getting only to where the anime has reached or a little further and then slowing down a lot that really depends and is different for every series. There is also often a lot of spin-off manga appearing for some series so they have completely different stories.

I personally would prefer going for the original source if I want to continue the story of an anime that has ended without finishing the story.

It depends. Some series originally manga some are LN. If the original source is LN, manga can be a little different from the source or can be way behind LN or never really finish. I personally always go for the original source.

When it comes to light novels/web novels vs manga, which ever came first is normally the best version of a story. This is especially true for Chinese stories that started as novels, where the mangas are almost universally regarded as butchering the stories they're adapted from. There are a few exceptions where adaptations actually improved on the original, but they're few and far in between.

If you're looking for some suggestions of good light novel stories (where their manga adaptations should be avoided), I rather enjoyed Desolate Era and The Rising of the Shield Hero (the laters link is for the original web novel. There's a light novel too, but like the manga it's a butchery of the original).

If the story is based on LN, then usually anime covers only around 60% of the story - only main plot points. And usually not even all of them if those those keypoints aren't important for current season (especially if the studio isn't planning to make another season of the anime - why bother adding useless things). Manga isn't much better - it adds details that you didn't see in the anime, but it's almost always rushed and story suffers from it (for example, iirc, anime and manga of Re:Zero didn't mention the fact that there's another person from Subaru's world, that Subaru met already).
So if you like the concept of some anime that you've seen - better check the original LN, manga would be worse. Same with anime based on manga or VN - better check the original source. There are exceptions of course, but usually it's like this.

The issue with light novels is finding a translation is hard for them because most people don't want to translate something so long. It's even rarer for them to have official translations. A lot of the time the translations are just script based too.

I used to read mangas. But now I'm more into Chinese Xianxia novels. Wuxiaworld anyone?

Usually manga are still adaptations of anime or lightnovels, LN/WN are still where the main stories are at. But if there's no LN/WN for the anime and it comes from the mangas then go ahead, manags are always still better than the animes.

Assuming we are talking about LN/WN being the original work, and it's later adapted into manga and anime. There are several cases of it being backwards, or multimedia mix (you need to read/watch all the content from different media to get the full story)

If you want to ask about the differences, you should ask with a specific series in mind. Usually, the manga tends to follow the novel closely in plotline and skip the long winded stuffs in the novel, but there are cases where the manga deviates or completely destroys the charm in the original work.

For example, Shinsekai Yori is originally a novel, adapted into manga and anime. Unless you have time to read the novel, you should watch the anime and skip the manga entirely, since the manga is overloaded with moe and totally failed to deliver the Mystery part of the story.

Another novel which I can remember on the top of my head is Another. There are slight differences in the ending of the LN/manga and anime, and some character survive in one version while dies in the other. No changes to the major events and plotlines, though.

The obvious problem is, you see, that mangas have much less text to translate from Japanese to English than light novels and as a result there are simply way more mangas available to you to read than light novels (that is if you don't speak Japanese, of course) cause it's easier and faster to translate.

For example: if you really liked some anime based on manga and want to continue the story you are almost guaranteed to find officially published version or unofficial scans online and it would be likely up to date with chapters released in Japan; however, if you enjoyed an anime based on light novel and wish to read the full story... well, good luck finding the translated version official or unofficial. You mentioned manga/LN for Re:Zero, the thing is the manga for it still hasn't covered the story in anime, and it has very slow release, the official translation of the light novel (published by Yen press) has only covered about 2/3 of the story in anime, despite the fact that they licensed it almost 3 years ago (~2015) and the anime got really popular in 2016. If you want to continue the story from where anime left off - you have no choice but to read unofficial translation of a free web novel(!) here: https://translationchicken.com, or https://www.zerotranslations.com