AD VS AP Items
Round 1: Mana Sustain
AP:
Morel is a god tier mana sustain item, however, ROA and TEAR are considered to be more troll if you build them. ROA offers mana sustain in exchange for taking damage, while burning off mana gives a small bit of HP. Tear reduces the amount of mana spent and can be upgraded from 15% to 25%.
AD:
Every single crit that you do, refunds 3% of your maximum mana. Thus- each and every minion wave can give about half of your mana bar back with less than half of an ADC build. As far as replenishing your resources go, the ADC champion pool have the edge of mana sustain with Essence Reaver. (Most of the time you will always be full on mana with this item. It gives crit + 30% CDR + High damage.)
Round 2: CDR
AP:
One of these items are universal, which can be a flexible pick for most AP champions:
however Ardent is built by supports and so is gauged for specifically that role with VERY specific champions. The golden sword is another niche item that offers CDR- in other words- you don't build it unless you're Kayle. New Banshee- it is a defensive item that is again- incredibly niche pick. You pick up the item if the enemy team has a Lux, Morgana, Blitz, Thresh, etc. Bane, another niche item. Pick it up if you're Fizz or TF. Zhonya- another niche item. Pick it up if you're up against Syndra, Zed or you're playing a champion like Rumble or Swain. If you combined these items, you have an opportunity for 80% CDR. Notice how the AP junglers don't even get CDR with
I guess AD's need CDR more. Than Mages. Do.
AD:
Right out the bat, AD have more items. A few are niche- but then again, that doesn't really matter when you're critting for 800 damage at the end of the game or 400 damage in the mid game. If you combined these items- you can have up to 130% CDR (obviously you can only have up to 45% but this is an opportunity to go more routes than your standard route.) Essence Reaver gives you so much mana, CDR and damage along with the CDR. Youmuus gives you Lethality- a huge damage spike along with a movement spent to either run away or stick to enemies. (Which has been nerfed countless times on supports, but AD is different hah-HAH.) Maw gives you an AP shield, CDR, Lifesteal, 20 AD, and even Spellvamp. Seriously, this item is pretty much a big fuck you to any Mid that blew their load on an AD only to watch it regenerate after a full Lucian Combo. AD jungle item has CDR on it, surprise mother fucker- Graves needs to waveclear faster than your Elise. Trinity Force, basically a bundle of stats that you buy at Target with a 200% AD steroid. Movement speed, extra damage, attack speed, AD, you name it. Iceborn. Let's be honest- this is Ezreals item. DD: "It would be cool if we made the only counter play to an ADC even harder to achieve if we gave the ADC an even bigger window to just lifesteal back up to HP after a full combo. Yes, we should do this right after the assassin update that makes assassins cosplay as a local serial killer- walking up to folk and giving them a handshake before killing them. Because they can't kill them right off the bat, that would be too quick.) Cleaver, damage - HP - Armor reduction and movement speed. That's basically what you need to know. Overall, the AD diversity has SO much more to offer.
Round 3: Coming When I'm Bored Again
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. I also wouldn't include 
. The ONLY reason this build comes up above 400AP is because of rabadans. If rabadans were removed the build would not be possible. It just lacks to much AP compared to the standard mage build.
for anyone wanting cooldown reduction for all classes.
Again more items, and more generally useful items. Across the items; lifesteal effectively being hidden extra health in a duel, having options between multiple shielding effects, damage reduction, and flat health. Triforce would be paired with the gunblade, but riot removed the ability power, leaving only the lich bane for magic damage dealers, which has mana and movement speed.