Use the following guidelines for choosing the best bokken for aikido or sword related martial arts. Since Japanese wooden swords are specific to certain schools (ryu), the following will help you narrow down options. When in doubt, check with your dojo-cho or select a general purpose bokken.
Aikiken Bokken- shown above is designed for Aikido but great for all around use, this bokken is good in a wide variety of practices and takes the best characteristics of older and newer designs. Available in 2 sizes: 40 inch and 42 inch lengths with optional tsuba (hand guard).
Shinto Bokken- shown above, the 40 inch shinto bokken is designed for around use and good for wide variety of martial arts including iai practice. Available in All Japan Kendo Federation specs for Kendo practitioners. Available with optional tsuba (hand guard).
Iwama Bokken- shown above, at 41 inches long, the Iwama bokken has a heavy, forward balanced, blunt tip, specific to Iwama style aikido. Excellent for direct, in-line takemusu practice where the path of the sword deflects strikes of partners' weapons. This bokken is largely oriented to the relationships of sword techniques to open handed practice originating at the founder's dojo in Iwama. It is not used with a hand guard.
Kenjutsu Bokken- shown above. As the curvature and hourglass shaped tsuka lends itself to many customizations, the kenjutsu bokken is available in various tsuka (haft - or handle) and blade lengths up to 45 inches overall. It is often used with long tsuka and best for martial artists seeking a long bokken. Available with optional tsuba (hand guard).
Yagyu Bokken- shown above. Thin, light and fast, this bokken 40 inch bokken is specific to the Yagyu Ryu but is also popular for specialized techniques adopted in other dojo. While the Kingfisher version is plenty strong, the design is relatively thin and will not affect the path of another wooden sword or staff in paired practice. Consequently, it's not intended hard contact but rather evasive and precision maneuvers. An excellent choice for one needing a light weight bokken. The Yagyu bokken is not used with a hand guard.
Katori Bokken- shown above. A specialized bokken of the Katori Shinto Ryu. At a relatively short 38.5 inch length, it has an unusual feeling of leverage due to the curvature—which is most prominent near the tsuka, and due to its relative stoutness in comparison to length. This subtle geometry creates a unique feeling of speed and presence so perfectly optimized that any deviation from the original design of the 1400s tends to undermine it. The Katori bokken is not used with a hand guard.
Kashima Bokken- shown above, this 41.5 inch straight, heavy, inertial bokken is always used with tsuba (hand guard) specific to the Kashima Shin Ryu.
Suburito- shown above, the 42 inch heavy basic swinging sword, contrary to outward appearances, is not intended as a club or even to develop strength but instead, to develop correct hip engagement and body positioning through multiple repetitive solo strikes. The suburito is not used with a hand guard.