Board Thread:Questions and Answers/@comment-23899747-20131021021522/@comment-108.9.15.103-20131125192243

They're equally correct. If you want word-for-word match, then Spirit World Warriors' Society is closer. (異界 = Spirit World, 士 = Warrior, 協会 = Society). Personally, I prefer Society of Spirit World Warriors because I think it sounds more like an actual society name. How many organizations have you seen with an s' in their title?