The manhwa. The manhwa goes off script from the novel in a few key places. So it's best to read manhwa only for this one (art slaps tho so worth it). You can technically find this part in the novel and continue from there but timeline events have been messed up a bit (from what other people have said a while ago)
For me it doesn't really matter since Ragnarok wasn't written by Chugong(author of Solo Leveling) but by a different author. You can consider Ragnarok as an alternate story and not a direct sequel. He wants to see how other author's would write the story. Chugong also revealed that he wants to write a direct sequel of Solo Leveling but didn't reveal other details regarding it.
Should I follow the Ragnarok story from the completed novel or from the manhwa? They both really differ in terms of plotlines and characters' situation and context.
I mean, at least in the Ragnarok story from the completed novel version, I already know that its OP good and has a satisfying ending.
But in this manhwa version, the fact that the apostle of Itarim is hiding among the allies and that Sung Suho still looks unaware of it makes me nervous. It feels like this will really stumble Sung Suho if he doesnt find out soon enough. Σ( ￣□￣||)