Southbound is the way to go without question. Ending your cruise in Vancouver is far superior to ending it in Seward. You have made a great choice. As far as port or starboard, it really doesn't matter. The Capotain will "spin" the ship so that each side of the ship will have equal viewing and when cruising the inside passage, both sides of the ship enjoy fabulous scenery. You can't go wrong either way.
Have a great cruise.