Both weapons are pretty good for boss fights, shield deals lower amount of dmg but hits 2 targets which can be helpful in boss fights with enemies besides main boss while sword is ideal for boss fights where the boss is all alone, it deals high amounts of dmg to melt boss quickly, of you're thinking of getting any of these weapons remember that they're only suitable for boss fights, will have hard time fighting groups of mobs, rather use Dozer axe for that nonetheless shield can be useful for small groups 4-6enemies since, if u time it correctly, can do up to 3 hits with drown effect during proc.