This decision is tricky. Ultimately the issue here is that neither of these groups have any representation on the Council right now- that means that both are likely to be out of touch with the leadership, disorganized, and ultimately underutilized and susceptible to low morale and feeling underappreciated.
That's pretty bad for anyone, but there are 10,000 Stirlanders. They form a full quarter of the human forces present. If we don't pick them they will have no leader to rally around (note how the Nordlanders and Ulricans united and aligned behind the knights but the Stirlanders and misc. human forces did not). This campaign could be seriously damaged by having those kinds of numbers left either leaderless or with a leader that they don't know and have faith in.
Contrast with the journeymen- yes, they'll be not listened to and underused if they don't have someone in charge of them... but they're journeymen. They've been explicitly judged by their Colleges to be capable of self-directing, determining the best course of action and moving independently to take it. Further, if they really need something and we're on the Council in any capacity at all nothing at all stops them from approaching us and bringing it up; even if we're commander of the Stirlanders if we say "gold wizards can do X, let him" no one is going to be dumb enough to say "that has nothing to do with the Stirlanders and so we're not going to listen"; all the people without representation on the war council have to be getting their orders from somewhere and the council has to get ideas about what those orders should be from somewhere.
Ultimately I think that if we want this campaign to succeed, taking command of the Stirland contingent is the right choice.
That said, if we're here for personal development or gaining Magister cred, maybe we don't give a shit if the Stirland contingent is disorganized and dissolves/deserts.