WD%, WD+, ED%, ED+. these affixes are calculated in this order.
the WD only affects Weapons that these affixes are on, either MH or OH, and can only be on Weapons.
the ED only affects Weapons with the same Element, but can be on any Item.
before you raise Item Quality or put Stat Points into Power, it is possible that +5000 WD is better than +100% WD. a Weapon with base weapon dmg of 4000 only gets 4000 more dmg with +100% WD, but you would get +5000 with +5000 WD.
ED% is figured after your WD, and ED+ is added last, so that is the least useful dmg. now if a weapon, even after adding WD, is doing less than 25,000 dmg, then +20% ED adds less dmg than +5000 ED.
now if you have the space, you would use all of these to improve your DMG. if you don’t have lots of space, then you need to figure out which gives you the best DMG possible.
looking at the Flintlock and Stats, it looks like the Base Weapon DMG is lower than 5000, so the +WD & +ED are doing more than the +100% WD. I read that the Power Stat is calculated after WD is, so that might be why the +100% WD Weapon is doing less.
here is a post that looks at how to calculate DMG. also, the Default Skill shows your BWD without any Skills in the way, Skills that do 100% WD with no Skill Points do the same DMG as Default, normally.