I think the biggest defense advantage comes at sea. In my experience, the defender ALWAYS has the wind at their backs. This is a major pain, because it means there's no real advantage to attacking at sea, no opportunity to outmaneuver your opponent.

IMO, who gets the wind at their back should be based on the number of stars the admiral has, perhaps give an admiral +%10 to his chance to get the wind at his back for each star he has over his opponent.

EX: If one admiral had 3 stars and another 4, the 4 star admiral would have %60 chance of getting the wind advantage, and the 3 star just %40. If they were BOTH 3 stars, then they would have an even 50/50 chance.