combat is better for units of equal strenght, since you win more battles. Drill is better for units significantly stronger, since you recieve less damage from fight you're sure to win anyway. Flanking is better for units significantly weaker (horsemen/horse archers), since it weakens the defender without losing the unit.
But in general, let's face it: once you've got eighter combat or drill, you're bound to get the other one as well, simply because you're not that likely to lose that unit anymore.
But in general, let's face it: once you've got eighter combat or drill, you're bound to get the other one as well, simply because you're not that likely to lose that unit anymore.