You should be using flame arrows by that point. I don't know about you, but I have enough resin to be throwing it away. Flint I have to go out of my way to get.

As far as I saw, slashing, piercing, and blunt all do the same vs the Draugr themselves, however blunt weapons are better against some other enemies in the swamp, like blobs and skeletons, so it's worth having a blunt weapon for those reasons. I primarily use an axe myself, and have a stagbreaker as a secondary weapon in the swamp so I can hit leeches easily.