Heroics & Shield AC IMO should be looked at individually. IIRC the parses i've seen do equate to 1ac in shield ~= 3ac elsewhere. It's been a while and i'm too lazy to do some digging for those parses.
TSW did parses with Heroics. Every 25 HAgi interval you gain 1% dodge (nothing in between) where as HDex gains parry & block with the same formula. I don't know how the HAgi gained through the shield affects the equation. However, if that extra HAgi pushes you over a 25 point interval, i would (personally) take the 1% dodge over a 5-10AC upgrade in the shield. Same goes with Hdex since it adds 1% to parry and 1% to block every 25 point interval.
is one of the URLs regarding the parses, is another (needs updated). Look about half way down for Yoda's posts. Most parses were done by Yoda but there are lots of other parses throughout TSW regarding heroics. I can scrounge up more if you'd like reading material.
In short, I'd go with the Shield of Blightwing, hands down.
On a side note, if you're a stickler for mana regen, the same rule applies to Hwis and mana regen. Hope this helps you decide and understand a bit more regarding future investments

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