War Brig is the perfect combination of speed, broadside count and cannons with complimentary cargo space. The ship is my preference for loot runs. If you're leaning towards a War Galleon, consider that it does have the largest cargo space and most broadsides, but the ship is incredibly slow. If you plan to use a War Galleon, be sure to have a crew that's ready to repair damages. War Sloop is another option, but I prefer this ship for SvS because it has extreme speed and is perfect for a well-leveled crew. The War Frigate is a ship that could be used for SvS and loot/mat runs, but it depends on the upgrades you give it. In terms of hull capacity, here is a simple run down:
War Sloop < War Galleon < War Frigate ~ War Brig.
-Hope it helps!