Crushing and bludgeoning weapons are good against plate mail. I think slashing's better against chainmail, but I'm not sure. I think the chain helps cushion crushing blows, but again, I'm not sure. Certain enemies can only be killed by certain types of damage. For instance, clay golems can only be destroyed by crushing or bludgeoning weapons.
If you can easily switch between crushing and slashing weapons, great. However, make sure you have potent magical melee and ranged weapons for everybody before you end Chapter 3. Make sure everybody has at least one proficiency star in a melee and ranged weapon if they can. (I know kensais can't use ranged weapons and not sure if archers can use melee weapons.) And be sure to have at least one weapon of +3 enchantment before ending Chapter 3.
