I've been looking at icecrown items and it seems to favor more armor vs avoidance at least with Emblems of frost items, I believe this is to Faster less hard hitting bosses and less avoidance in icecrown, I havent seen the discusssion around but whats the good ratio to swap this out at,
You are getting hit still when armor kicks in, just makes you get hit for less
Avoidance avoids the damage entirely Both are purely from a Physical damage perspective fight. Stam wins for the magic fights but these armor pieces have more stam and then more armor but less avoidance
Armor lowers the amount of damage taken from Physical damage attacks (which is a lot of fights) and you could look around Tankspot forums for some of the math on how much Stamina each point of armor is worth. Avoidance is just avoidance and it is not static like Armor is. Avoidance is a % and it always will be, however Armor is always there (excluding armor reducing debuffs) and this is very similar to the Stamina vs. Avoidance argument.
Armor is considered effective health and it is superior to Stamina once you reach certain amounts. I know that a lot of tanks have switched to armor as their main Effective Health stat (gloves armor enchant, the armor trinket from triumph badges etc.) and it seems to be very good.
On a purely magical fight armor and avoidance are both irrelevant so there really isn't an issue as far as magic damage fights go.
As far as balancing avoidance and armor goes it looks like the ICC fights are still keeping effective health the dominant tank gearing method and as such stick to Armor and Stamina.
TLDR: Armor is effective health and static, avoidance is not. ICC looks to require EH still so go armor when you have the option.