There is now a LessWrong group on fimfiction, to let LWers on fimfiction find each other and collect stories that might be of interest to them. (That? Which? Grammar Nazis, help!)
"That" if you're a grammar Nazi; either one if you're a professional linguist or mere native speaker of English. :)
"Which" is used to add parenthetical commentary to a noun. "That" is used when the relative clause is restrictive: the clause disqualifies some members of the larger set of objects to which the noun refers. Since LWers are not interested in all stories, but only a special subset, you are correct to use "that".
I've joined! Step two: actually start publishing.