Is one definitively better than the other? Certainly people have their own personal preferences but is one system superior the other? In short I would say no, there is no superior, they are different. For me it’s the old apples and oranges analogy, they’re different things altogether.
Armor Class was developed along with what is generally regarded as the primogenitor of Table Top RPGs and Role Playing Games in general. It is iconic to Dungeons & Dragons and one of the bits that has survived all subsequent editions. I do find it a little strange when looking back on 4th Edition that they did not change to an Armor Soak paradigm given so many of the other revolutionary changes to D&D in the edition. If there was a time in which it would have been acceptable to fiddle with the mechanics of AC, 4e was it. Armor Class functions that wearing armor makes it more difficult for an enemy, monster, or trap to damage you.
Armor Soak was a later development. New RPGs brought new mechanics, many no better and often worse than the original mechanics developed through D&D. But only with a lot of work can you turn coal to diamonds. Soak was a mechanic that came out of the many varied mechanics. Damage Resistance was around in the games and campaigns using the AC system but it was something different, a special ability or the creation of magic for the most part. The idea to remove armor completely from the equation of avoiding an attack and instead using it solely to express resisting damage of a successful attack was revolutionary. Why it works so well is because it makes common sense. Wearing a suit of articulated plate mail doesn’t actually make me more difficult to actually strike with an attack, rather the opposite, but the armor is able to protect me from damage of the attack. A bulletproof vest may help to keep me from taking a bullet in the torso but it doesn’t cause bullets to magically deflect around me. It takes some of the lethality out of the blow. A fractured rib and a deep bruise is a small price to pay in exchange for not dying due to small arms fire.
So that is the fundamental difference between AC and Armor Soak. The former is a measure of how difficult the target is to damage and the latter is a more granular study of when hit how much damage does armor keep from harming the target. That is why the latter cannot actually function on its own as a mechanic. Armor Soak does not encompass how difficult the target is to hit in the first place. Armor Class on the other hand squishes both those ideas into one mechanic. A successful attack roll against AC immediately tells everyone the attack landed well enough to cause damage to the target. Armor Class does not take into account the difference between a deflection, glancing blow, and a total miss; essentially both end with the defender not being damaged. Of course one could easily make such a determination with some rudimentary math. A PC with a +2 Dex Mod to base 10 AC, wearing Leather Armor for another +2, and a Buckler for +1 has a total AC of 15. Attacker rolls an 11, Base AC + Dex is 12, he probably dodged. Roll a 14, the PC probably knocked the attack aside with the buckler or the attack just grazed his armor.
The “Roll To-Hit” layman’s term is probably a leftover of the old THAC0 days. That was considered a “To-Hit” roll, though newer editions are straight attack rolls. D&D does not distinguish between attacks that do no damage and complete misses. And depending on the system you play with Armor Soak mechanics, neither does it. Without a minimum damage option for successful attacks heavy armor can be impervious. Back when I reviewed the playtest of Sword & Shield, I noted the lack of minimum damage being a problem. There was the ability for one creature to be completely unable to harm another creature due to maximum damage of the attacker and the armor soak of the defender.
Where Armor Soak outshines AC is in the latter’s accessories. AC is a simple, a succinct and useful system if a bit more abstract than its Armor Soak counterpart. Unfortunately that ease of use is lost when all the other bits are added to the system. Damage Resistance, Temporary HP, and Regeneration being the main culprits. Now after determining that a target has been hit hard enough to sustain damage it can reduce the raw damage, then the damage must first eat through any Temporary Hit Points before dealing damage to the target’s actual HP. Then later, depending on the situation, the target will regenerate a certain number of hit points or create new temporary hit points making the dealt damage almost immaterial. It is a slippery slope of combat mechanics that need to be tracked and tweaked throughout the entire combat. It can be a serious headache. By the end AC and its sundry companions make combat more cumbersome than Armor Soak.
Armor Class can also seem a bit redundant in the d20 system that makes use of Fortitude, Reflex, and Will defenses/saves. It would be natural to conclude one of the three could be subbed in for Armor Class on almost any attack and work in concert with an Armor Soak mechanic.
One thing I do find superior with Armor Soak is that when a PC is hit the armor always feels like its working. You can see its effect as it mitigates the raw damage. Armor Class on the other hand can feel frustrating as a string of high rolls on the attacker’s end can make heavy plate feel useless as it flies over the defender’s AC. When an attacker rolls a 19 on the die, heavy plate is about as useful as an armor made of rainbows and tufts of dandelion.
So is one method better than the other? I would say no. My favorite option would likely be an AC system that included some minor bits of Armor Soak mechanics. Reduce some of the temporary HP and damage reduction madness. I would say maybe give medium armors a ‘Soak’ of one and heavy armors a soak of two. Armor is one of the places where simulation generally wins over gaming philosophy. Heavy armor is a curse, it ups a creature’s AC but at the cost of reduced speed and all Str/Dex based checks. This is a double punch for PCs as they are usually wearing heavy armor because their Dexterity is not great, they don’t need added negatives to checks. The same goes for sleeping in armor. As a simulation yes it makes sense that sleeping in heavy armor is uncomfortable and impedes rest. But beware night attacks as your fighter with his brand new AC of 11 based on his poly/cotton blend boxer briefs because donning armor takes exponentially longer than the fight itself. I think with all the flak PCs have to deal with for wearing medium and heavy armor they can be rewarded with armor that routinely soaks a meager few points of damage.