... fchan, because
- on fchan, if you try to post the word "e621" anywhere, you get the deliberately vague error message "posting not allowed". if you try to circumvent this, for example by saying "type 126e backwards into google and search for it", then the mods will delete your post. on e621, on the other hand, you can post as many references to fchan as you want.
- on fchan, stuff gets deleted randomly & without warning. on e621, there's a list of deleted posts in your profile, with reasons even! and you can ask for something to get undeleted.