if you haven't started doing any of the martial arts, I suggest you should do kendo first. Through my experience, the kendo movements are more active and hard to coordinate. Iaido, on the other hand, is calmer. If you learn kendo properly, it is easier to do iaido.

The Japanese martial art - Iaido Iaido places great emphasis on the correctness of forms (kata), the precision and efficiency of movement, and mental focus. Practitioners of the art - Iaidoka - use real katana (or blunt training swords - Iaito - which are still quite dangerous), so it is primarily a solitary art.