Ahoy Captain. FireBrand upgrade is worst upgrade there is as your it gives small cargo hold, no armor bonus and fire ammo is not the strongest ammo there is , so your ships broadsides will not be strong.
Storm Chaser is Fast good Turning Ship with mega Strong Broadsides , but no armor bonus , so it is is Kamekazee Ship.
Fortune Hunter Gives very big Cargo, good Speed ,armor Turning and looks Royal Posh,but not strong broadsides.
I would recommend Fortune Hunter if you like big Cargo capacity , if not then Skull And Bones.
Skull and Bones small cargo but strong broadsides, armor and no decrease in speed and turning.
CopperHead is to slow turning and poor speed