Please use the search function, this dead horse has been beaten so hard I'd be surprised if there was any flesh left on the bone.
There will never be an answer to this question / debate beyond "GEAR FOR THE ENCOUNTER". Now what exactly does that mean?
It means you break down the bosses abilities, decide what is likely to kill you, and gear for that. If it's magic damage like Sarth 3D, Stamina usually wins out. If it's a ton of physical damage ala patchwork then armor is probably the winner once you reach enough stam that a botched heal wont get you killed, but avoidance may be do-able. In fights where your your EH threshold is really low, and the majority of damage is physical and avoidance set (with enough life) is probably worth trying. If it's somewhere in the middle, well you gear somewhere in the middle. The key is to decide what is likely to kill you and gear so that event doesn't kill you.
The reason many people simply stack stamina is Stam is always there, it always works. It's not a chance, it's also the current trend due to boss design in WotLK.
Now we're (being tankspot regulars) are freaking sick of the EH vs Avoidance debate (which is essentially what your question boils down too), it's raged for years; as the encounters change the trends change. If you want to read more about I suggest again the search box. There are tons of pros and cons for each side, more then what will get re-hashed here.
If you want just my personal opinion then here it is: Get bonus armor where you can, gem and enchant for stamina, and don't wear eitriggs oath, it's about the worst trinket in the game. Wear stamina trinkets, or an armor and a stam trinket. Don't ever prioritize avoidance; in wotlk it's very easy to get "enough" avoidance even without prioritizing it.