My previous post was lengthy and I was wrong about a very critical fact about mitigration scaling. Thanks to Phreak for setting me straight.
TL;DR Both health and mitigation scale linearly; prefer health early on because it helps against all incoming damage sources. Itemize not only based on your opponent's comp but also which of your opponents is doing well.
In depth:
I previously stated that the mitigation becomes less efficient over time, but I was wrong. Both health and mitigation scale linearly. 100 armor provides you with 50% mitigation. If you have 1000hp it will take 2000 physical damage to kill you. 200 armor gives you 66% mitigation, meaning it will take 3000 physical damage to kill you. Every 100 points of mitigation gives you 100% more effective health from that source.
I maintain that health should be favored over mitigation because it allows you to absorb all 3 types of damage (physical, magic, true). Purchasing 100 armor does nothing to protect you from an ap carry, so building out a solid base of hp is a great way to soak damage from both sides and adding mitigation contextually.
To demonstrate the principles that effect your build order, I'll discuss jungle Amumu and cover two scenarios. The enemy team has a balanced comp so I will account for both types of incoming damage. I always start with Philosopher's Stone and Heart of Gold. The passive gold helps my late game itemization, the Philo allows me to give blue to the ap, and the HoG establishes early health for tanking. I use 0-21-9 masteries, armor yellows, and mr/level blues, move speed quints and magic pen reds.
Ad carry is doing awesome, ap is meh:
- Randuins or Frozen Heart. The Frozen Heart is very popular right now, mana, cooldown reduction, armor, and a debuff aura are very compelling. Note however that these stats will do nothing to protect me from the enemy ap. If I expect incoming magic damage, the Randuins will protect me from both sides and give me the aoe slow.
Ap carry is doing well:
- Aegis/Negatron. The aegis gives me everything a tanks need and is extremely cost efficient. With health from the HoG and Aegis, a Negatron will substantially mitigate incoming magic damage. I would likely go back to buying armor at this point because the utility of the big armor items tends to beat out the negatron items.
I've found it very challenging to succinctly describe the driving forces that optimize your itemization because there are so many variables at play. Watching pros is a good way to start building a body of knowledge around when to build what. Pay attention to what's happening in the game, watch the order in which items are picked up, and actually talk through the decisions with yourself (or a buddy) to determine how wise the choice was. It's worth noting that the mathematically you can calculate the maximum effective health for a given amount of gold, however this is of limited value because in reality you are limited to six item slots and items give you specific stats that may not follow this curve. Additionally for legendary items you are paying for things other than the base health and mitigation stats so it's difficult to say what item will treat you best based on simple models.