Why is 'A' better than 'B' in two shields I'm looking at

Shield one:
http://www.wowhead.com/?item=25078
"of the beast"
+13 Strength, +13 Agility, +20 Stamina

Shield two:
http://www.wowhead.com/?item=25074
"of the champion"
+11 Strength, +17 Stamina, +11 Defense Rating

Tankpoints, which is of course a warrior addon, rates the first shield vastly higher than the second.

Some of this is obvious, armor and block and a little stamina.

But you're also losing defense rating. Looking the two over has got me wondering, just how much defense rating can I lose in favor of other things before it becomes a bad idea?

How do I sit down and calculate that sort of thing out?
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Take the one which is looking the best


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Just look at it.

First one has more stam, and agi which is dodge. Agi plays a major role in your dodge % before 70.
Second one offers less stamina, and some defence which will give you less avoidance than the first.

Hence what tankpoints says.

Btw, tankpoints is not a warrior addon, it's a generic addon. It just tells you how good a item is for "general purpose". It actually assumes that you are already uncrittable.
Anyway, crittabilty aint rly a issue until you get to heroics. Nothing hits for enough to bother you until then.
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Pretty sizable chunk of armor and 9 block value difference.

To figure out how much def you "need" you have to take into account the mob level and figure out its crit chance against you, and thus how much reduced chance to be crit you need from defense.

I wouldn't worry too much about defense and being uncrittable while leveling. Its a great stat for avoidance but the level 63 shield offers a lot more effective health.

The first one has a lot of armor (more than 1% mitigation at that level) and agility for huge dodge bonus and more stamina. It's a clear win-win situation

And tankpoints is a bit... biased to warriors since it gives strength a bit more credit and it has no spelldamage/spellhit :p
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Agi is only a little bit worse than defence rating for pure avoidance, and it has some armour too.

It doesn't increase your block rating though, but Agi isn't a bad stat for paladin tanks, its pretty similar to dodge rating.
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Yeah it's pretty clear to me that the first shield is better, I'm just trying to break it down element by element to help me with making future judgments.

To which I've been very unclear on 'how much of 'x' can I give up in exchange for 'y'.

I've been working under an impression that defense rating is the most critical item on the list, but even if so, by how much has been unclear to me.
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Look at the base defense of the items you are comparing (and inherent block value in the case of the shields). The items with the higher base armor have a higher Item Level. When at a loss for which item to take while leveling, just go with the higher iLevel. It won't matter that much in the long run.

ps: if your level 63, go grind out your sporregor shield and don't look back
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WoW is a giant math problem, and you have the wrong answer.