As long as Armor Class is a thing, I prefer descending armor class to ascending. I’ll explain.
In terms of ease of use, ascending is better. The math is easier. It’s easier to add than subtract. You know this is true intuitively even if it’s not strictly true for you. You see it among your friends. Ascending AC says, “add your attack bonus to d20 and compare.” So that’s a real easy mechanic. It’s easier than subtracting an armor class from your THAC0 and comparing. So in that sense ascending is better - it’s easier.
But there are two things that make descending AC better than ascending.
The first one has to do with bounded accuracy and the arms race that happens with skills and stats in more modern editions.
In D&D games prior to 3E, there were only 20 (or 21) armor classes. Just 20. There was AC 9, which was the worst, up to AC -10, which was the best. You couldn't get any better than the equivalent of AC 29 in an ascending system.
But in the ascending systems of 3E and subsequent versions, your AC can go sky-high. You can have an AC of 40, 50, 100. What does that mean? It means that if someone wants to have a reasonable chance hit you, they need an attack bonus of some astronomical number as well. And the same is true for the DM's creations: Super high AC and super high BAB (base attack bonuses).
Not so in descending. There's little reason for super inflation of attack bonuses because even an ancient red is only going to get up to a little less than AC 29.
The second thing that makes descending AC is related to the first, but it's more of a player-side aid. That is, with only 20 steps to the AC scale, you can chart it very easily and put it where everyone can see it. It can be reduced to a chart rather than a math problem. Here is the entire range of AC versus attack roll results in Mythical Journeys. This chart is based heavily on 0D&D:
Players
Monsters
That's it. No math and very simple charts. Click to enlarge as usual. Obviously the part that goes from -1 to -10 is missing here but it would be trivial to show it as well.
So you can actually make these math problems into charts for the math-challenged. After using them for one session, they will be really easy to use.
Of these two reasons, I would say the bounded accuracy of descending AC is more important than the ease of its use because it kills off the arms race. It also means a guy at level 4 is not all that much better than a guy at 1, and not that much worse than a guy at 8. The difference is in resources (hit points, money) rather than in personal skill. And that keeps the characters feeling more realistic and less like superheroes.
