When do i build each of these items? Obviously one is AP and one is AD, Dark Seal being for champs who aren't mana hungry early game (Sorry Amumu) but when would i build each item? I heard someone say that Dark Seal builds into Magus Soulstealer or something but i can't find that item.
Axew1 votessomeone is trolling you friend that item does not exist.
Better Jungler1 votesThese are really strange to compare to be honest.
And its a strange place to post this (player recruitment)
But I guess you're recruiting players for help?
Regardless, Dark seal is great for when you think you're going to crush your lane. Poachers Dirk is incredible (after its new buffs) when you're not afraid of the enemy jungler, and feel confident invading him.
Dark seal will stack, then build into an item called MEJAIS SOULSTEALER
and gives you a massive amount of ap for the price, but its risky, because if you dont perform well, its not worth building. It's also cheaper than dorans ring, thus you can get one extra pot.
Dirk is great if you see the enemy jungler is busy bot lane, and your champion can clear his opposite side jungle quickly. With the new buffs, you can get all 4 stacks without a cooldown in between (turning it into serrated dirk) which gives you a nice new burst of damage. (Raising your ap, while lowering enemy armor)
TL:DR
-Confident you'll get kills/assists, on an AP champion, and plan to build mejais
-Jungling, not afraid of enemy jungler/enemies collapsing, AD and looking to build 
ItsMaybelline1 votesMb, it auto filled player recruitment.