Ballistas are definitely more useful for the war against the Ironborn, however, it won't be utilized until the war begins. Since the duration is only 2 turns, it can be done during the Spring when we don't have to keep watch on our expenditures for buying food stockpiles if this Winter is prolonged. Hence, "cheaper during winter."
Guard Training, on the other hand, provides a quality increase in a small number of All our vassals' men, increased loyalty (for intrigue as well as possible levy call bonus?), immediately applicable (patrolling, etc).