Even in a game with a flat probability "curve" (such as a game using a d20) there really are a lot of variables, making it next to impossible to say with any authority whether a +1 defense or a +1 damage reduction is best.
In AGE, using 3d6, the simple 5% probability per step leading to an uniform number is replaced by a curve, meaning you'd need integrals for an exact answer. In layman's terms: if the enemy has an easy job of hitting you (or already finds it difficult) then better Defense is most certainly not worth the effort, since each point in either direction corresponds to such a small percentage.
If, say, the enemy hits you on 6+ and you upgrade your Defense, now the enemy hits you on 7+. But the reduced success rate is marginal compared to the case where the enemy starts out hitting you on a 10+. (The difference between 10+ and 11+ is significantly larger than the difference between 6+ and 7+ since 3d6 creates a bell shaped probability curve).
This means that unless the enemy lives in the in-between-extremes lands of "easily hitting" and "rarely hitting" you are better off with an armor increase (compared to a d20 game).
Since most Dragon Age foes start out with a great attack value they're always going to have an easy time hitting you, unless you max out your Dexterity.
Meaning that unless your Dexterity is stratospheric, giving you a Defense that's actually decent against the foes you expect to face, you shouldn't waste your resources adding to it. Put bluntly, a Defense of 15 is hardly better than a Defense of 12 if the foes sport attack bonuses of +9 or thereabouts.
Only if your Defense is already 20 or so is it worthwhile adding more points to it, since Defense 21 is such a significant improvement over 20 (assuming that the enemy used to have to roll 11+ and now needs to roll 12+).
Another problem with defense (compared to armor) is that those hard-to-get increases lose their worth again when you face weaker foes. If you have Defense 20 and you face a monster having a +5 attack bonus, he already needs to roll 15 on the dice, meaning he will almost always miss. Increasing your Defense further (to 21) will not meaningfully change this fact: yes, the hit chance will be further reduced, but only by a few percentage points.
Meaning that Defense increases is only really worthwhile in a small band of opposing attack values. But when it does shine, it shines brightly: in the best case, a one-point Defense increase lowers the attacker's hit probability by 13%, which, given an average damage of 15 points, means almost two points of damage.
Of course, increasing your armor rating by 2 points does the same job, and much more deterministically. Of course, that won't help you against penetrating damage...
In the end, I'd say it boils down to a couple of fairly generic points:
* if your games master throws a lot of weak and/or hard opponents at you (but few in-between ones), go for armor
* if your games master takes time attempting to balance encounters so that combatants need to roll 9-12 on the dice to hit, go for defense (this is not the default case, I'd say - you normally find it much more easy than this to hit or be hit)
* if your games master throws generic foes at you, whose attacks aren't "special", go for armor
* if your games master tries to outsmart your minmaxing
by adding foes with penetrating damage or "save or die" attacks (such as poison or paralyzing magic (that still needs to hit you physically) or other stuff that sidestep Health and armor), go for defense
* if all your foes use magic against you, ignore both Defense and armor, and focus on stuff like Constitution and Willpower instead
But also, what has already been said,
* if your Dexterity sucks, you are probably better off by ignoring Defense completely
* if your Dexterity rocks, you are probably better off by continously finding new ways to boost Defense further (up to a certain point, after which you should still maintain a decent armor rating)