Cerys is the OBVIOUS person the devs wanted you to choose.
She's intelligent, wise, mature, progressive, diplomatic, everyone talks about how awesome she is and has the makings of a near perfect leader.
Hjalmar is a bumbling retard who's childish, impulsive, and makes every stupid decision a man could make. The only reason anyone would ever choose him, is because they either already chose Cerys on another playthrough or because they don't want a woman as a leader.
The game is heavily stacked in Cerys's favor.
Smart, wise, mature, great leader?
Or dumb retard who will destroy the isles by engaging in every war he can find?