It depends on how you wish to play the game. If you are seeking to be more the inbetween kind of player (DPS / Tank) then you could go for medium armor.

If you want to play full tank, you better go for heavy armor.

It's not about which is better, it's about what you wish to play.

You shouldn't ask what armor should you use for some particular class, but for role that you want to fill in your group.

If you are DD(Damage Dealer) then you need to find out which line of abilities deals most damage proportion. These lines can be magika-based (class abilities, destruction staff abilities) or stamina-based (two-handed sword, dual wield, bow, one-handed and shield)

If you think that most damage comes from stamina-based abilities then you should use Medium Armor in order to maximize your damage output.
If these are magika-based abilities then consider of using Light Armor.

Remember that maximum amount of stamina/magika affects damage done with abilities that use given attribute.

You still can use Heavy Armor as DD (melee-chars will benefit from Juggernaut passive), but IMO you will do less DPS (although I didn't test this)

For example...
I like toe-to-toe fights so I'm gonna play melee DD. All I want is to do maximum damage to a single target.

I will skip one-handed and shiled line as it definitly a Tanking skill line,
so I have to chose between Two-handed and Dual Wield.

I prefer Dual Wield as I find Two-handed skill line more specialized for Area of Effect (AoE).

Twin Slashes - powerfull Damage over Time effect (DoT) that I will constantly keep up on my target.
Whirlwind - if I need some AoE effect
And Flurry as my main damaging ability.

There is also a bunch of great passives and one of them is Twin Blade and Blunt. It gives 5% chance to crit with dagger so I think there would be great synergy with Nightblade Assassination line skill Hemorrhage.
I will pick Pressure Points for even more crit chance.

So I'm gonna choose Nightblade as my class.

I will use class abilities like Haste and Mark Target abilities. These gives some constant buff and their effect don't scale from my maximum amount of magika.
Also I want to use my Assassin's Blade to finish my targets off. I won't use this abitity much as it becomes effective only when my target's HP is below some threshold (if I remember correctly - 25%).

As you see my magika-based abilities effects either situational or don't scale from maximum magika (some constant effect).
On opposite - I will use my stamina-based abilities constantly so I will benefit greatly from it.

I want to put all attribute points to stamina and I want to use Medium armor because:
1)I want to have high stamina regeneration (Wind Walker)
2)I want that fancy haste bonus (Agility)
3)I'm getting more benefits from crits (Dexterity)

I'm not pretending that this is best possible build, I just wanted to show how I came to conclusion on which type of armor should I use.

Hope it will help.

Okay, focusing more on the 2H skill tree or the DK skill trees?

Ah, AF / 2H. You might do well, then, in a mix of light and medium, actually, heavier on the medium, less on light. If you're not in a 'tanky' role, you don't need the armor of heavy or its passives.