the destroyer is tiny, while the battleship is super huge. hopefully my battlecruiser model gets accepted, and bridges the gap between small and big as a medium size and agility.
having said that, the kusari ships rely on speed to survive and are thus nimble fast and have small powerplants, while bretonian ships are built as tanks, being slower but with more firepower, armour, and bigger powerplants.
the kusari destroyer is VERY weak if you get behind it, since it has less backward facing turrets. granted, due to its size and agility its a bit hard, but remember to use CDs effectively, and as a team, and chances are the destroyer would be blown up more efficiently.