Your build is very similar to
mine, and so far it's working really good.
As for your question...
[wowitem=25896]Powerful earthstorm diamond[/wowitem] is the way to go imo, especially since it stacks with Nerve of Steel giving you a rather chunky 15% stun resist and 18 STA is a pretty massive chunk of hp as well.
[wowitem=25899]Brutal earthstorm diamond[/wowitem] is a cruel joke. The +3dmg to weapon really doesn't amount to much dps wise (As this gem isn't even a good dps choice - [wowitem=32410]Thundering Skyfire Diamond[/wowitem] and [wowitem=32409]Relentless Earthstorm Diamond[/wowitem] are better from that point of view) and the stun is a 1 second stun with a 1% proc rate. In other word, it doesn't work often and when it does, it doesn't last long. If you're mace spec, you'll hardly notice the extra stuns.
[wowitem=25895]Enigmatic skyfire diamond[/wowitem] is a good runner up, but imo, it's a better setup with Mutilate - since it stacks with Fleet Footed and Mutilate is a lot more crit dependant then AR/Prep. Do note that pvp gear usually is pretty stacked on crit too, so it's not like you'll be hurting for that extra 12 crit rating.
And for what it's worth, Skyfire diamond are usually more expensive then Earthstorm, narrowing the choice even more.
Once you reach over 10k HP (without counting gems), I'd highly suggest to load up on +16ATK for your red sockets and getting the 3 PVP gems (20atk, 10 crit, 10atk/5crit) as well. Use 4resi/6sta for your remaining yellow sockets.
Aside from getting 4resi/6sta to replace my 4crit/6sta gems (in neck and bracers) and replacing my 10crit by the 12crit BoE JC yellow (in shoulders), I'm mostly going to stick with the same gem pattern as
I have right now for S3.
Survability is important, but we still got to deal damage.
Edited, Nov 24th 2007 1:24am by Tyrandor