Roleplaying is fifty years old this year. There are some arguments/discussions/debates/jihads which have been ongoing for most of that time. Today I will resolve one of them once and for all!
AC or DR?
D&D, and many other games have armour make characters harder to hit. It is often said, “This is ridiculous, armour doesn’t make you harder to hit, it just makes those hits less dangerous.” Before you can stab reply they go on, “What is far more realistic is if armour reduces damage after you are hit.”
This feels logical but the majority of RPGs which use damage reduction end up with problems where combat slows to a crawl as well-armoured combatants trade attacks for 1, then 2, then 0, then 0 again, then 1 damage. Sometimes DR flat out breaks the game as some character builds become nearly untouchable. Also actually applying the DR is another step in the process and nobody wants to spend longer on combat do they?
So AC feels cheap, DR slows games to a crawl and makes balancing a challenge, no wonder this argument has been going on for decades. Clearly there is no good answer…
The Good Answer: Low DR
Something I like about AC is that if an attack gets past the armour it still matters, because it still does full damage. Whereas with DR if you have DR 3 plate armour, that d6 spear is only ever going to do 3 damage maximum. I kind of feel that once the spear is through the armour, you’ve got a spear in your guts and the fact you are wearing armour doesn’t really matter anymore.
With Low DR armour reduces damage up to a limit and then does nothing at all, you take the full hit. So if you have DR 2 and you take 2 damage, your armour absorbs it and you can carry on uninjured. If your opponent rolls a 3 for their damage though, your armour is breached and you take the full 3 damage.
Here’s a more complete example which I think shows the difference well:
So now armour protects the way people expect it to, but hits are still dangerous and best of all, we don’t need maths to subtract armour from attacks, it either cancels the whole hit or does nothing at all.
Ominosity 
The answer is hit points by location. So that 3 damage can be nearly disabling as it takes out a limb. Yes that means one more roll but you could roll the hit location die at the same time you roll to hit as long as it is a unique die. only works for gritty low level games but it works well.