In terms of raw numbers (weight and hydration), purified water ekes out the advantage over the sealed waterskin, but you get a sealed waterskin right when you start playing, while purified water isn't available to craft until you've unlocked the stove at level 29. Pound for pound, purified water is still better than the Deep Drink waterskin, while it's inferior to the Khari waterskin, and you can't make a straight comparison with the Healing waterskin. Of course, waterskins can be filled anywhere you can find water, while purified water requires a crafting process at a base.