Magic heavy fights = double stam trinkets
Everything else = stam + armor trinkets
The reason for this is that armor provides a very predictable amount of damage reduction to every physical attack. Consistantly taking less damage is always a very good thing, avoidance is not consistant and is very unpredictable so its often viewed as with alittle bad since bad luck it can very easily get you killed and wipe the raid. For magic heavy fights the armor bonus is fairly useless since magic damage ignores armor so double stam trinkets are best used. More health creates a larger buffer between how much damage you can take and how much you actually do take.