"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.