http://crucible.samanna.net/viewtopic.p ... sc&start=0
The sticky post on shield ac has some good discussion on AC vs HP. There's really 2 parts to the discussion. This is how I understand it and think about it, I am sure others will chime in with their opinions.
First, there is a softcap on AC where returns diminish, going from 990ac to 1000ac is a much bigger gain than going from 1460 to 1470, for example.
Second, there is the point at which your AC overcomes the magic AC number for the mob you are fighting, it's somehow related to atk but I don't know the details. When you reach this point, the mob starts hitting you for minimum damage very often.
Since you play a warrior and AC Is their deal I am sure you understand that part anyways.
I have about 100ac more than you and I am sad to say that mobs in DOD/RSS/etc can still pwn me like a wet napkin if I am unlucky with getting missed. To pickup 100ac to match me, you'd need to drop at least 4-5 aug slots into that, which would be at least 200hp in trade. You're talking about picking up 500ac though, which is no small task, unless you are looking at getting into heavy highend raiding gear (which doesnt look like it from your magelo) you will need to devote all aug slots to AC. If you were a raiding shaman and could actually get your AC up to 2200+ where it would actually make a difference against mobs in places like RSS/DOD etc, then maybe you would want to weigh the AC portion more heavily than you do now.
So, to answer your question directly, I'd take a 25ac aug over a 50hp aug only for my shield slot only. Sadly I think there may only be one such aug in game that is usable by shamans. In any other slot, I would not personally use AC at this time. In terms of gear, I weigh it by the piece. I personally go for the combinations that will maximize my focus effects, HP, other effects, resists, mana and then still keep my AC at or around 1600. If it drops below to 1580ish..no big deal.
Anyways, just my opinion as someone who has similar stats/gear.