You need to be a member of one of the organizations in order to enter a trial. Most clubs affiliated with one of the organizations will require you to also join the national organization in order to be a member of the club. So I guess you could say at some point membership in one or the other becomes mandatory.
Since you are just getting started there really is no reason to join either one at this point, but if you stick with SchH you'll eventually have to. People most often end up picking which organization to join based on which one has the most clubs, and thus most trialing opportunities in their area.