build armor or health first (locket of the iron solari)

[placid island]

I'd say it depends on the enemy botlane

Then go armor

[balmy kiln]

Depends on when you back and if you need the cdr early. You might also flex armor parts into boots depending on how the lane unfolds, so generally it's more versatile to build Aegis.

[calm granite]

You need to take into account armor affects minions damage and if they have an AD jungler it'll also bring more value

Not that plain simple to calculate item values