Genrally speaking the STH series is for freeskiing, and the Z series is for all mountain riding. The Z series has a self adjusting toe piece and a slightly less durable but lighter heel piece. They are ideal for people who ski the groomed runs, or small park riders. The STH Driver series uses a fully adjeustable toe peice (best on the market) and a much more durable heel piece. Where it gets a little confusing is there is an STH 12 Oversize which uses an oversided version of the Z toe piece. It is much ,much more robust then the standard toe of the Z series and uses the same heel as all the other STH bindings. The OS is ideal for sombody who wants a slightly cheaper binding, or sombody who does not need a bombproof toe.