It is somewhat difficult to quantify the +1 AC in an easy to summarize way, because the percentage reduction depends on how easy it is for the enemy to hit. In the extreme case where an enemy hits you only on a 19 or 20 without the +1, then +1 AC would actually reduce incoming damage by 50%! The value of that +1 also changes depending upon the gear you wear, and whether or not you wield a shield.
For this reason, I am not really confident when I say that the +1 AC is better than the "bugged" version of GWF, but I do lean in that direction. Damage buffs seem like they are easier to come by than AC buffs in this game. It wouldn't be all that difficult to change my mind though.
