Waltorious: Well, it depends on how much. Since the Arcane Platemail has double the DR, but only slightly less AC, then yes, it's better. But if it had double the DR and half the AC, it would probably be roughly equivalent (at least in terms of damage per time).
If you have a more or less balanced armor, or wear other equipment specifically to compensate for its deficiencies, then it isn't really obvious how it works. But if the two stats are far enough apart, then you have something like this:
1. Low AC, high DR: You get hit more often, but take less damage. This means you can take more hits in a row without having to heal, and once you do heal, your opponent has to spend time bringing your HP down
again. You can easily get a good idea of your durability this way, and focus more on combat tactics than on keeping yourself alive.
2. High AC, low DR: You get hit less often, but take a lot of damage. This means that a strong enough enemy can take out a large chunk of your health in one hit, and you might not be able to heal all the damage at once. If you're then hit again immediately because your enemy is luckier than you, your health gets even lower, so you get stuck wasting healing items instead of fighting.
These are extreme cases, of course, but since there are enemies that tend to do a lot of attacks per turn (hello, Wolf Cave...), and thus decent chances to score a hit simply due to the attack rate, the lack of damage resistance can become painful quickly.
And, technically, Dodge skill prevents getting hit as well, so it kind of "enhances" AC on its own (and there's no reason not to learn Dodge, particularly if you go for training; who doesn't like seeing enemies kill themselves on a crit miss?). Plus you get AC bonus from high dexterity; on the other hand, for most of the game there's no "natural" way of increasing DR if you're not a half-ogre or a lunatic.
