I see this armor/allres question a lot. However, when comparing to pieces of gear, keep in mind that it's a % increase interest.
If you have 630+ allres and 3k or so armor, then armor is ABOUT 1.5 times as good as allres (after you've /10 of course).
Example, you have 3573 armor, I'm going to assume 700 allres. (Reasonable for a monk, wiz, or WD).
Your effective health multiplier from these two things is 1/(1-3573/(3150+3573))*(1-700/(315+700)).
Looking only at strength, we can make it look a little simpler by changing the equation to 1+3573/3150, which is 2.13. Resistance is similarly 1+700/315 which is 3.22. We want to change these by the largest FRACTION possible, since they multiply. Doubling one doubles your EHP. Now if that takes 3150 armor, or 1200 resistance, armor's better. Let's look at what 10 more armor would do as compared to 1 allres.
(1+3583/3150)/2.13 = 1.00149, or a .15% increase in survivability.
(1+701/315)/3.22 = 1.00098, or a .098% increase in survivability.
Or that 10 armor is roughly 1.5 allres. So, if you compare to things, divide the armor by 10 mentally, easy enough, and then think that you'd need half again as much allres to equal that.
So, looking at 245 armor, that's 24, cause we're mentaling it, and that becomes 36 allres. Don't attempt to balance your res and armor at some value, that's just silly. It's MUCH easier to get allres or specific resists on pieces of gear than it is to get armor. It's better to compare piece by piece.