I recommend joining all, not much help in choosing I'm afraid, but I can tell you what each one gets you.
The Japanese Sword Sword Society/ US gets you 6 Newsletters a year, 40 to 50 pages each. We also have a by mail lending library for books that may or may not be up and running now, and the same for sword related videos. Also, your membership helps pay for translation and publishing of Japanese texts (like the new Sue-Koto book just out).
The Florida Token Kai publishes 2 or 3 times a year. Current volumes are reprints of the NBTHK's English journal, "Token Bijutsu", the best information in English on the sword.
Northern California Japanese Sword Club publishes a monthly newsletter, "To Ron", which is usually between 20 and 30 pages. Your membership helps to stage the San Francisco sword show every August (same for Florida and the Tampa show in February).
NBTHK American Branch gets you 12 issues of "Token Bijutsu" in Japanese, with a partial translation available on line. It also gets you membership in the Japanese (main) branch and access to their shinsa services. Members get free and advance entrance to the special displays they've been putting on at the 3 shows in the US and a members only display and Kantei session at 2 of those shows.
Those are the ones I belong to, and recommend wholeheartedly. Perhaps someone else can write about the NTHK and any other organization I'm not familiar with.
Grey