Someone can probably be more definite.
Pirate
Ore loot appears to be topped out at bronze or gold (forget which).
Wood loot appears to be topped out at ash or Yew (again, forget which)
I believe you can't get barbed leather from a pirate
Chance for white cloth dye tub is lower.
Merchants
Any ore possible. AND, colored ingots possible.
Any wood possible
Any leather possible.
greater chance for white dye tub.
Not sure how the foodstuffs, raw material for making cloth, and other specials (like the booze and ammo) are different.