I have been answering a ton of "wich armor shold I choose" questions, both ingame and on the forum. So far nobody has ever disagreed with what I said, so I asume that those are true enough to be written down once for future reference. The Rules of the game and the Roles (state 1.0.5) are:

1. Class does not say anything about role
Telling me you are a "level 34 Templar" is about as usefull as saying "level 34 Character". There is some synergy between certain roles and classes as there is between some roles and races but every class can play every role.

2. What defines Role are the Weapon and Armor you choose, with armor being the slightly more important part (it's easier to switch weapon then armor).

3. The armor provided by armor is a secondary concern. Armor's primary purpose is to give bonuses that support the role via thier skilline passives. If you are not investing into armor skills you might as well run around in heavy regardless of role (because then you at least have some decent armor rating).
The only way to improove Spell Resistance at all is via the Passives.

4. This is only a basic guide. It will not hold up in the most extreme MinMaxing Scenarios. Like very late game and PvP/AvA. But it will still give you the right ideas.

Light Armor: If you are heavily using any class skills, Destruction Staff, Restoration Staff, Mages Guild or Undaunted Skills this is the Armor to go.
The skills drop magika costs, improove magica regeneration, spell crit and spell penetration. It also has the most impressive amount of spell Resistance bonus (via passives and the active).
Healer and Spell/Class based DPS is what these are all about. If I have any reason to give a character light armor, it often also has the healer role (so easier to find groups for Dungeons).

Medium Armor: Any weapon skill other then Staffs and 1 handed with Shield, this is the armor to go. Plain and simple this is non-spell, non-class DPS armor. It boosts Skill Critical chance, Stamina Regeneration and Attack Speed.
It also has some buffs for Stealth, Sprinting and Dodge Roll Costs making it a good idea for everyone valueing stealth or mobility.

Heavy Armor: This combined with 1 Handed and Shield is just plain the Tank armor. It has the best base armor rating of all armors and passives that further improove the Armor/Spell Resistance, Health Regeneration, Healing Received and Block Cost.
There is a boost to Weapon attack power, but before you choose heavy as DPS armor you have to be certain that this one cancels out the lack of the stamina regen and Crit Chance Provided by Medium Armor. Few things can make healers life as hard as a DPS that is not dealign enough damage agaisnt the proper targets.
It might also be possible to use Mage Guild Equilibrium skill to make a heavy armored spellslinger/healer.
Be advised that high Armor alone is not what makes a good tank. Proper direction of aggro and use of Block/Dodge is almost as important as high armor and easily healability.


The "Overcharge and Heavy Armor" problem:
Once you do select a proper tank built (maybe with some class buffs, racial buffs or higher quality armor) you will run into "Overcharge" on both Resistances. Basically it means you are pretty good already (for your level) and should consider investing into other things instead.
Overcharge is often called the "Softcap". For Armor and Spell Resistance it appears at about 33% mitigation*. There is also a hardcap at 50% mitigation*, but due to the softcap it is not likely to ever be reached.

There are three basic ways to deal with Overchage on Armor (and thus when tanking):
"High values are only for those commited": Basically this says "I don't care for overcharge, I just keep adding too it". There might be less return for the investment, but there still is a return. As long as you do not reach the hardcap more is still helping.

"Mix and match Armor": You only keep 5 pieces of heavy and take two of light/medium armor. This approach unlocks some bonuses from the other armor line while still retaining the Block cost reduction and most of the other passive effects of the heavy.
Especially with the high amount of usefull class/mages guild skills for tanks and the much bigger boost to spell resistance the "5 Heavy + 2 Light" combo seems to be common in lategame.

"Time go use something else": This accepts that your armor might not be the best or even approach the hardcap, but is good enough. Heavy armor gave you enough armor already that you don't need to waste further traits, enchatments and skillslots on boosting it anymore.
Heavy Armor freed up ressources for other things beside armor rating, so look for decreased Crit Damage (Impenetrable Trait), Elemental Resistance and Max Life/Magika/Stamina boosts. All of those make the tanks and/or healers life easier. It is closely related to the mix and match armor.

What is a spell and what is a skill/weapon?
This is the information most often said. I am not sure how accurate it is*:
If it uses Stamina, it is considered a weapon/skill. Wich leads to odd effects like "boost melee damage" to improve Bow use. And the bash damage done by staves.
If it uses Magika, it is considered a spell. It uses spell crit and is resisted by spell resistance.
If it uses both Stamina and Magika, it is considered a Skill. Stamina seems to go "first" and once it found a match noting else is considered.

Relative Armor values and Skill Bonuses per Armor Type (Armor values from white Veteran 10 according to crafting menu):



Light
Medium
Heavy


Chest
146
220
269


Feet
128
192
235


Hand
73
110
134


Head
128
192
235


Legs
128
192
235


Shoulders
128
192
235


Waist
54
82
100


Total
785
1180
1443


Armor value relative to heavy
54,40%
81,77%
100%


1st Passive
# * 1/2/3% less magika cost
# * 1/2/3% Weapon Crit
# * 1/2/3% more Armor
&
# * 1/2/3% more Spell Resistance


2nd Passive
# * 2/4% more max magika
# * 2/4% Stamian Regen
# * 2/4% Health Regen


3rd Passive
# * 4/8% more Spell Resistance
# * 3/6% less detection radius
&
# * 4/8% less stealth cost
# * 0.5/1% Weapon Power


4th Passive(5 set bonus)
5/10% higher Spell Crit
5/10% more attack speed
10/20% cheaper Block


5th Passive
# * 3/6% Spell Penetration
# * 2/4% more Sprint Speed
&
# * 2/4% less dodge roll cost
# * 0.5/1% more healing



(this was autogenerated BB Code, not sure why it is not being processed by the Forum).

Hope that helps and I did not get anything seriously wrong.

*In those cases I am only repeating what others have said. I cannot take any guarantee for accuracy and of course it might change in later version.