Does adviser choice matter

For the player, armor reduces the damage taken by physical attacks by 2 for every armor point in the campaign, or by 1 in multiplayer.

Armor on monsters reduces the damage taken by 1 per point.

Armor is applied after damage multipliers (like %-Attack, Crit, etc.) but before damage reduction from % melee defense.

The formulas for damage taken are:

For monsters: [WeaponDmg * (1 + Attk%) * (1 + CritDmg%) * (1 + FlankDmg%) * (1 + BarrierDmg%) * (1 + GuardDmg%) - Armor *(1 - ArmorPen%)] * AbilityMultiplier%

For the player: [BaseDmg * (1 + CritDmg%) * (1 + FlankDmg%) - Armor * 2(only in solo mode) *(1 - ArmorPen%)] * (1 - Defense%) * (1 - ElementalDef%)

Source: http://dragonage3.wiki.fextralife.com/Attributes

There is no bar or other mechanic like in the previous Dragon Ages that tell you how far along you are with their approval. For the most part, you have to go by how they address you when you talk to them outside of combat, such as at Skyhold. For example, if Cassandra is your friend but isn't romanced, she'll often start the conversation with 'my friend.'. Or Varric, who will usually open up with a quip or joke of some sort if he's your friend.

There is also a visual representation on the 'cards' of the character when you go to pick your party to head out into the world. While I'm unsure if it's tied to their approval or simply to acknowledge that I have done their personal line of quests, it seems that only the characters that I'm friends with have their cards changed. Take that as you will.