In tanking there are a few traits you need to know about to make a choice according to your needs. I will explain the relevant traits, what they do and when you should use them.
Shoulder, Hands, Waist, Feet, Body, Legs, Helm
Reduces cost of block
Sturdy reduces the cost of block. This trait you will use onall your armor. Sturdy has become the only useful trait for tanking. Since the Morrowind update there is simply no other trait that will give you a significant advantage.
Nirnhoned/Reinforced -> Shield
Increases resistances of armor piece
Why do we use armor increasing traits on the shield only? Because of a passive in the lady champion point tree called 'Shield Expert'. This passive gives your shield 75% more armor. This stacks multiplicatively with a nirnhoned or reinforced bonus. Ultimately nirnhoned is a little better than reinforced (about 50 armor points on cp160 gold). So if you can get nirnhoned, do it. Otherwise reinforced is fine.
The following traits all apply to one-handed weapons worn together with a shield.
My personal preference on the main bar. Gives you that little bit of extra resistances. It might not seem like much, but in my opinion it is still a good trait to use on the main bar.
Chance to gain a small amount of extra ultimate
If you feel like you want an extra bit of ultimate regeneration, decisive is a good choice. Especially for dungeons where there is not warhorn rotation. Though I do not recommend this trait in trials as it is simply not needed.
Increases enchantment effect
Infused pairs well with a crusher enchant (reduce enemy resistances). The buff looks nice on paper, but in reality any other person in the group that is running a crusher enchant without infused will simply override your infused crusher. So ONLY use this if you are sure all other group members with this enchant also run infused.
Increases healing done
I like to run this on the bar where I have vigor slotted to get that extra bit of healing. Great for an off-tank in trials or a dungeon build.