Wanderlei made this well thought out post on the forums, which is a nice distillation of the viewpoint he has consistently presented as a counterpoint to the heavy stamina/armour theory. Even if you don't agree 100% with him, this is definitely worth your while to read.
---Avoidance saves mana only if the healers are healing reactively.
I can't tell you how often I see this, but it's so wrong that I don't even know where to start. Any healer who heals in a 25-man environment should be used to casting constantly, and canceling unneeded heals. There is precisely NO benefit to healing someone who is already at full HP, so there is precisely NO reason not to cancel that heal before it goes off. This is neither hard nor uncommon - its been the standard way of healing in raids since the Molten freaking Core. A tank who avoids more attacks will allows his or her healers to cancel more heals, and therefore save mana. THAT SAID:
The point of avoidance tanking in 25-mans is not mana conservation!
That's our second piece of misinformation. Mana conservation is great, but generally unneeded in 25-mans. The true idea behind avoidance tanking is, due to the way avoidance scales, as opposed to the way that effective HP scales, your odds of avoiding potentially lethal damage are better than your odds of surviving it in the majority of situations but - and I stress this greatly - not in all situations.
Stamina provides a static amount of HP gain which becomes proportionately less relevant the greater the damage per hit taken. For example, 36 stamina or 360 hp is highly significant when you are being hit for an average hit of 500 damage, because it will usually allow you to survive and additional hit. When you are being hit for 5000 damage on average, then it is drastically less probable that 360 hp will mean the difference between living and dying. And I want to stress that this is entirely a matter of probability: neither avoidance, nor stamina is less based on luck than the other. In both cases, random or uncontrollable variables (mob damage variance, and avoidance rolls) produce an amount of damage which has a probability to kill the tank based on the frequency and amount of healing the tank recieves, and the outcome of those random events. If mob damage was constant: for example, if a raid boss always hit for exactly 6000 damage, then you would probably want to get 18001 buffed hp, and then stack avoidance. Since boss damage is not constant, the amount of HP you want for a particular encounter cannot be so easily determined. However, mob damage is not so varied that there is not a general area you will want to aim for beyond which the advantages of additional stamina are much less unless you can get so much as to survive an extra hit on average.
On the other hand avoidance scales faster the more you have. To illustrate this, we consider how often you are HIT, not how often you AVOID. With 30% avoidance, as one poster suggested, you have a 70% chance to be hit. With 50% avoidance, you have a 50% chance to be hit. 50 is roughly 71% of 70 - what this means is, you are being hit 29% less frequently with 50% avoidance than with 70% avoidance, not 20% less frequently as one might suppose. To illustrate how this scales in an accellerating manner, consider also the difference between 50% avoidance and 70% avoidance. 30 is exactly 60% of 50, so you are being hit 40% less frequently with 70% avoidance than with 50% avoidance. A much greater improvement than going from 30% to 50%.
Now I said that this keeps you alive by preventing potentially fatal damage spikes rather than (usually futile) attempts to survive them. Rarely as a tank are you facing potential death from a single hit, and it is usually more than two hits from a raid boss to kill the MT. So you have many chances to avoid a hit, such that consecutive hits either do not occur, or that a crushing blow is prevented, in order to prevent the death.
For example, the other night I was tanking Magtheridon, and his average hit on me was rougly 4500 with cleaves hitting as high as 7500. I had about 18k HP, so it would have taken 1 cleave plus three hits, or 1 cleave, 1 hit, and 1 crushing blow. Now Magtheridon does not actually hit fast enough to crush a tank who is spamming shield block, so we'll ignore that second option for this example. Now in order to survive the three hits plus 1 cleave, i'd need rougly 21,000 HP on average. Which means I would have to gain 3000 HP from what I was using. Now I have the gear to get about 20k when I want it, but 21k is out of reach for me currently. The point is however he has to hit me four times in a row for this to even be a potentially threatening scenario.
Now if I were Mr. StamStack and had my 20k HP gear on with maybe 45% avoidance, his chance of hitting me those four times in a row would be 0.55^4 or 9%. Well a 9% chance is not that bad, because if healing is remotely there I'll live anyway. But with my avoidance gear on I'm at about 65% avoidance, and the chance there is only 0.35^4 or 1.5%.
The point is that the situation which might kill me if healing fails to come through at that moment for whatever reason is SIX TIMES more probable in my stam gear than in my avoidance gear. SIX TIMES is a huge difference. In fact, with my avoidance gear, the chance of even being hit three times in a row is less than the chance of being hit four times in a row in my stamina gear.
So many people seem to fail to understand this HUGE benefit of avoidance in raid tanking. People stack stam in order to survive the "frequent" damage spikes they encounter tanking in raids: but it becomes a self-fulfilling prophecy. Because as an avoidance tank, I know from experience that the type of damage spikes I hear other tanks describe happen to me so rarely, and the funny thing is I still "somehow" manage to survive them more often than not. They survive them more often than not too and probably more often than I, but they also have six times as many to deal with, if not more, which means they die more often to it as well.
The benefits of stamina are also subject to random chance.
I really tire of people pretending that stamina is always a bonus but that avoidance is a random chance. That's simply not true. Stamina is only useful insofaras you would have otherwise died. I cannot begin to count the number of raid bosses I've fought where, even as an avoidance tank, my HP has never dipped below 1000. Well what that means, is that for all those attempts, I had 1000 HP that was providing precisely NO benefit whatsoever.
People are quick to blame not having enough stamina when they die, but how many review combat logs to see just how much HP they had when they took that 8k hit? Well I for one do, and let me tell you, of all the times I die resulting in a wipe - and let me tell you that it's not a frequent occurance, I usually die well after the wipe has been called or at the enrage timer, or when all my healers are dead - but as I was saying, in less than 20% of my deaths for which my death was the direct cause of the raid wipe would an additional 2000 HP have saved me. The exact figure is that an additional 2000 HP would have saved me in 7 out of 38 such wipes since the start of TBC raiding. Not only that, but all 7 occurred in Gruul's lair. There have been precisely zero against Magtheridon, in SSC, or The Eye. The last time I died to Magtheridon I was hit with a cleave for 7813 when I had 379 hp. I doubt I could have made up with that in stamina, but perhaps if I had 5% more avoidance, I never would have even been at 379 hp.
The fact is, that adding stamina gives you a probabilistic chance to survive based on the damage range and random damage roll of the boss you are tanking. As a consequence it is just as much subject to chance with regards to increasing your odds of survival as avoidance is.
While avoidance can adversely affect threat generation, most threat issues are due to poor play, and there is still a large threat margin to work with if everyone is performing properly.
The difficulty of holding aggro over "good" dps is greatly overstated. I'm not going to say that all of our DPSers are the best, in fact we might have one or two who have been seen out-dpsed by a hunter's pet. But that having been said, the fact is that it's a simple matter of mathematics. In my avoidance gear I usually put out in the area of 900 to 1000 threat per second - suppose 950 on average. Well for our friendly neighboorhood warlock to generate 950 threat per second, and assuming that he has salvation and is talented for 10% threat reduction, he would have to do 950/0.7/0.9 or over 1500 single target DPS. If he used Soulshatter intelligently, he could no doubt squeeze several extra 100s into that before he was generating 950 threat per second. Well we have a good warlock or two, but not that good lol. So it's simply not an issue. And until I start seeing lots of logs on wow metrics with guilds full of DPSers pushing 2k DPS, I'm not going to buy the threat argument.
This is not to suggest that you should give up lots of stamina for small amounts of avoidance
That is just as dumb as doing it the other way around, and can have even more serious consequences if your HP drops into two-shot range. Filling up your sockets with +8 dodge rating gems is not the way to go. For the most part, use +12 stamina and if you can get a socket bonus by using one +8 dodge rating gem in place of one +12 stamina and the socket bonus is either stam or defense or dodge rating then I'd suggest going for that. And if you have two interchangable pieces for the same slot, then you should strive to max the avoidance out on the one which is more avoidance-oriented, and max the stamina out on the one which is more stamina-oriented, which brings me to my next point:
No tank who is doing everything they can to help their guild succeed uses just one set of armor to tank everything.
When I tank Gruul, which I don't much anymore, but when I do, I max my stamina out just in case the DPS sucks for whatever reason. Because Gruul is a stam fight - the bigger hits you can survive, the better your chance of winning. For void reaver, it's max avoidance to dodge knockbacks and because he has no spike damage at all. For morogrim, well his damage is not threatening in single or double hits but only in large chains which are more easily broken than survived, so I go heavy avoidance, but not to the point of sacrificing 200 hp for something like 0.4% avoidance. The point is that you should be analyzing the fight and gearing appropriately. And if you die, you should be analyzing the combat logs to figure out why, and adjusting your gear appropriately. And you should be keeping old gear which gives you an advantage in one area in case that would come in handy at some point. I mean, anyone who's main tanking should already know this, but for all the armchair tanks out there, that's how it works when you're doing a good job as MT.
---At this point, you should be realising that this is really disagreeing only with respect to what constitutes an acceptable amount of effective health, which goes back to the words given here, in the section on The Great Debate. Having read both, you're now armed with the knowledge you need to make effective gear choices for whatever you face.