Can anyone tell whether you can have two different stones? Like one guardian and one Assassin depending whether you want one for DPS or tanking?

The +40% only applies to healing potions and mana potions, not all heals. The +40% usage is more suited to mana users, many tanks would rather pop an ironshield pot than a health pot every 2 minutes.

Because it's defensive rating, not defense (skill)

What makes it good is the mitigation fact combined with the potion effect. A druid tank has zero use of the potion effect as they cannot use potions while shapeshifted.

However, I would rather pick this trinket because of the huge overall mitigation than a stamina trinket in situations where I really want to make sure to survive or make it easier for healers to heal me. In other sitautions, sure, a stamina trinket is better.

It really much depends on the encounter in question. If tanking on a melee boss with a fast attack speed (1.5 or faster) then this trinket can be godly because it does increase your overall mitigation so much while 500 more health is easily chopped away when bosses start hitting that hard.

In any case, a completely dodged/parried/blocked attack means zero damage in most cases which imo, is overall better for survivability, which still makes this trinket one of the best in the game in terms of mitigation because no other trinket (or item) has such high def rating.

wthbrosiff59, having more def than the minimum is always helpful to me as a Warior tank, it increases your chance to parry and such, meaning I have a higher chance to not get hit. I'd say this trinket is good for any tank class that also uses pots. =P Although I desided its not worth learning alchemy just to get it, if I had alchemy already I would definatly strive to get this trinket.