It's good to be ruler if you have a competent general to depend on and vice versa. If either the ruler or the general is bad/inactive, then it is best to be the judge. Judge can function even if the rest of the realm is really lame.
My favourite position was when I was ruler/general hybrid of Assassins on the Colonies. The rest of the realm was very inactive, but through my own initiative, as master of both diplomacy and warfare, I started two wars and won them both. It brought life into the Assassins, and got them involved with the events going on around them. Some may say that the Assassins were better off being a neutral haven for infiltrators, not concerning themselves with the world at large, but I disagree. At the point in time which I did this there were only a few infiltrators yet many warriors with nothing to do. So, for most of the realm, I think it made their lives more exciting and purposeful.