All games systems are abstractions, like maps. They reduce an intricate, exceedingly complex and chaotic world to simple, easily handled abstractions. The degree of abstraction varies from system to system, but compared to the initial abstraction of any game from what that game represents, that range is exceedingly narrow. Even so, people have differing tolerances within that narrow band.
Today I'm babbling about two opposed techniques for dealing with abstraction - Frontloading and Backloading. Statistically, they are equivalent, but perceptually, they are very different. The difference lies in where certain operations occur in the game.
Frontloading - This is the practice of putting certain statistical abstractions into the down-time or non-play phase as opposed to in-play. An example of frontloading is armor making a target more difficult to hit.
Backloading - This is the practice of putting certain statistical abstractions into the in-play phase as opposed to down-time. An example of backloading is armor absorbing or reducing damage.
Statistically, these examples are pretty much the same. Over repeated instances, the average damage sustained by the target with either method, all other things being equal, should be very similar; with the similarity increasing with more instances, and thus a better data set. Pereptually, as any gamer will shout passionately, they are totally different!
What armor does is reduce damage to the target. That is so obvious it almost goes without saying. Therefore backloading is the right answer! Correct? Well, it's the obvious answer. Backloading increases handling time in-play. Each time damage is sustained, it must be reduced before applying it to the target. By frontloading this reduction, we can decrease handling time, making for faster combat.
Frontloading damage reduction directly is really awkward, due to the variety of weapon/armor interactions, and the random nature of damage. A more elegant solution is to change the chance to hit in the first place. "Heresy!" you say. "Armor doesn't make you harder to hit! it makes damage less lethal!" True, but we are abstracting here. By making armor harder to hit, we reduce damage indirectly over time. The average amount of damage sustained by the target can be reduced because less hits occur on the average per round, because even with the same amount of damage per-hit, by lowering the average amount of hits, you lower the damage sustained over time. This, however, is not intuitive to the gamer. We have, in effect, redefined "Hit" from "anything which touches the target" to "Anything which touches the target and does damage."
So in general, frontloading decreases handling in-play but increases perceived level of abstraction. backloading decreases perceived level of abstraction but increases handling in-play. The choice is up to you, but both are valid choices.