Maybe it's covered, I did not really see a good explanation, so I'll add mine to the mix.
Question: More armor makes you more killable?
Answer: It depends, the short version is that lower combat defence makes you more killable, while higher AR makes you less killable.
The damage you take depends on two variables, base damage and degrees of success. Well, critical hits is a third, but that's a general "you're screwed" add-on to damage that already should be fairly impressive. And it would be utterly painful to calculate so I'm ignoring it.
At the core, you have the fighting test result versus your CD, for each 5 steps above that is hit, base damage is dealt again.
Before going further, I will use padded armor as the reference. It has AR 1 and AP 0, thus you do not reduce your chances of avoiding being hit.
When an attack is successfully made against a character in padded, it is either 5, 4, 3, 2 or 1 step away from an additional degree of success. Unless he already have 4 degrees, but I will make my calculations on the assumption that this happens rare enough to be ignored.
If said character was wearing soft leather, his defence was 1 lower due to the AP. If the fighting result would be one step away from another degree against padded, we are talking an additional degree of success in soft leather. In Hard leather, this would be the case if it was 1 or 2 steps away. Essentially, compared to padded's AP of 0, each point of AP adds a 20% chance of an additional degree of success.
Essentially a person in soft leather takes on average 20% of the Base Damage of the weapon (B) before applying AR. In general terms, before AR is subtracted from the damage received, compared to padded, a character will on average sustain an extra damage of 0.2*AP*BD.
Armor penalty of 5 translated into one free degree of success, compared to armor penalty of 0.
Expanding the formula above, compared to padded, a character takes 1+0.2*AP*B-AR extra damage on average.
If the result is negative, then this means you'll take that much less damage on average compared to padded, while a positive result means that you take that much more damage compared to padded.
For examples, let us take a look at the best armors in their AP category:
Soft Leather: AR 2, AP 1: 0.2*B-1
Breastplate: AR 5, AP 2: 0.4*B-4
Splint: AR 7, AP 3: 0.6*B-6
Brigandine: AR 8, AP 4: 0.8*B-7
Half-plate: AR 9, AP 5: B-8
Full-plate: AR 10, AP 6: 1.2B-9
You can input them in a program such as this: http://my.hrw.com/math06_07/nsmedia/too ... hCalc.html
I made a screenshot showing:http://img826.imageshack.us/img826/7614/screenieth.png
red=soft leather, green=breastplate, blue=splint, purple=full plate.
From the graph (remember that negative is good in this case), we can see that of the selection, full plate is best up until base damage 5, after which splint is best until base damage 10, at this point padded is best, but breastplate also becomes better than splint. Soft Leather is better than padded until base damage 5.
The second screenshot for the heavy options only:http://img33.imageshack.us/img33/7705/screeniemw.png
Red=splint, green=brigandine, blue=half-plate, purple=full plate.
In this case, the math turns out that at base damage 5, all of them are equally good. While before base damage 5 the higher the AR, the better the armor, while above, the lower the AR, the better the armor.
Note on critical hits:
There's a higher chance of getting them against you with lower combat defence. Lower CD by 1 and the range in which an opponent might land a critical hit goes down by 2. Critical hits may increase base damage, which always makes lower AP armors better. So AP increasing the chance of having a critical against you, and the most common result of a critical (increased base damage) pushes it further down the line. It might result in as much as an increase to 1+0.25*B-AR, though I don't think it'd go as far as .25.
But against base damage 5+, yes, more armor makes you more killable.