I've been debating which yellow seal is better for dominion for a tanky build.

1. Defense - .15 armor per level for (2.7 armor at level 18) total of 24 armor.
2. Vitality - 1.08 health per level for (19.44 health at level 18) total of 175 health.

If it were SR I'd value defense rune more, but considering this is dominion and all of the AD have the 22% armor penetration (12% from aura + 10% from mysteries). The 24 armor will get reduce to 19 armor giving it less value, then again dominion usually have more AD champion then AP so it's still logical that even with the diminished defense, it is still a better stat then health.

So which one is better in your opinion?

I personally have a preference for the armor per level yellows and MR per level blues for tanks. Can't give you any mathematical comparisons, but I think the resistance runes are better than HP for tanks.

Got it, so they stack multiplicatively. That means (1 x .88) x (1 x .9) = .792 of the armor so closer to 20.8% instead of 22%?

In that case 24 defense x .792 is 19.008 armor. As apposed to 24 defense x .78 which is 18.72 armor which I chose round up 19 to begin with.

Too many warwicks and poppies. It's dangerous to go alone, take armor seals.

You shouldn't be scared of getting more health just because some champions do %damage.