Well, the web novel is basically the prototype version of the light novel.
The storyline of WN and LN remain mostly the same up to the Cal Mira arc, where the story starts to diverge. Stuff occurs differently and new characters are introduced. After that, The Spirit turtle incident seems to start like in the WN, but then goes off in a totally different direction when the time comes to finish it. Then you get a totally new arc, where you meet a bunch of new people and Glass' character gets fleshed out and a whole lot of interesting stuff happens. I've only read up to volume 8, so dunno what happens afterwards.
Either way, I definitely recommend the LN version. It's a definite improvement over the WN.
Oh, and fennel, can't really tell how far it is compared to the WN at this point. After Cal Mira, the story starts to contradict the WN quite a bit (in a good way, things make much more sense), so you can't say for sure. For instance, Raph-chan gets introduced in Volume 8, rather than the WN, where Raph-chan appears much later (with a different origin story too)
