Now that we have this straight, i'll go directly to my point in the form of a simple statement (and from there, you can decide if you want to the read the entire thread justifying this statement).
ARMOR BONUS IS (ALMOST) COMPLETELY IRRELEVANT UNLESS YOU'RE ON MAX LVL (60).
Why do i state such a thing? Well, lets look at some numbers:
First of all, your armor bonus is used to calculate your damage reduction using the following formula:
Theoretically, better armor will aways mean a better defense. But HOW MUCH BETTER, is what you should ask... Lets have some real life example:
One of my medium lvl range characters is a lvl 30 monk right now, so i'll use it as an example. Now lets consider some facts about my specific monk:
The final result (in realistic terms, judging by what i was able to buy on real AH) is something around 1000 armor (lets even say i was able to double my armor in that case, so lets make it 1120 total bonus).
Conclusion: Renewing my entire equipment to DOUBLE my armor increased my resilience by 27%. AND THIS IS THE BONUS FOR MONSTERS AT THE SAME LVL THAN ME. FOR STRONGER FOES, THE FINAL IMPROVEMENT IS EVEN LOWER THAN THAT.
Now to the big revelation of this thread and the real reason why you should simply ignore the armor values until you're lvl 60:
Unlike Armor, Vitality bonus raises your HP on a LINEAR fashion. 1 VIT point = 10 HP points.
Now lets go back to my Monk. Lets say that, instead of renewing ALL my items for better Armor, i exchange i SINGLE PIECE OF ARMOR (my Chest Armor, in this case) for a new one with the same amount of armor, but with a VIT bonus of +115 (i literally just bought this item from the AH for 5000 gold).
+115 VIT means + 1150 HP. What raises my final HP to 2000.
With this SINGULAR piece of crappy armor, i was able to raise my resilience on 135% what means it takes more than double of the hits to kill me. AND IT DOES NOT MATTER IF THE FOE HAS HIGHER LVL THAN ME. I'LL ENDURE 135% EXTRA HITS ANYWAY.
Finally, if a go to auction house and do the same as i did on the above example (renewing all my equipment for VIT Bonus, instead of ARMOR BONUS), i got a final VIT bonus of +920 (This is not a hypothetical number, i'm buying stuff while writing this)... And as above, i only bought affordable items for my lvl (everything around 2000 ~ 5000 gold AND i'm not buying weapons with bonus, just armor). This gives me a new final HP of 10050.
Conclusion: Renewing my entire equipment to raise my VIT bonus increased my final resilience by FREAKING 1082% [YES, A THOUSAND AND EIGHTY TWO, MORE THAN 10 TIMES], NO MATTER WHATS THE MONSTER LVL.
So the in the end, the big conclusion is: THE BIG NUMBER IS A LIE. LOOK FOR EQUIPMENTS WITH BETTER VIT BONUS AND COMPLETELY IGNORE THE ARMOR BONUS AND THIS WILL AWAYS BE BETTER FOR YOUR DEFENSE WHILE YOU'RE NOT ON MAX LVL.
And then you may ask; What about inferno? Is this still true on inferno?
The answer is: As long as you progress on the game, the 10 hit-points per vit point become less and less effective. BUT the VIT bonus for high lvl equips also gets higher and higher, what helps to balance this loss.
On inferno, both Armor and Vit Bonus are important. I cant tell which one is more important cause i dont have a max lv char yet, but i'll surely do some investigation when i get there.
So, my final advice is: Forget the armor bonus. If you want to stay alive, look for items with VIT bonus. The higher, the better. Armor is almost completely irrelevant if you compare the bonus given by the extra vit.