Idol of the Corruptor currently drops from General Vezax in Ulduar and will be obtainable for 19 emblems of conquest as soon as 3.2 comes out - and everything will drop emblems of conquest, at least. The proc is 153 agility for 12 seconds, and it has close to a 100% uptime since it can be refreshed with another mangle. If there is an internal cooldown it is less than 4.5 seconds.
The Idol of Mutilation costs 25 emblems of triumph, which are only obtainable from the Coliseum raids (10 and 25 man) and daily heroics. The proc rate for lacerate/swipe appears to be quite high and it seems to have close to a 100% uptime as well, and there appears to be no internal cooldown. The proc gives 200 dodge rating.
With the nerfs to agility and dodge rating, here's what each item does before diminishing returns factors in:
200 dodge rating / 45.2 = 4.42% dodge
153 agility * 1.1 (kings) * 1.08 (sotf + motw) = 181 effective agility /47.84 = 3.799% dodge
These are with the new 3.2 nerfed agility/dodge multipliers, in case you were wondering.
So the Mutilation idol gives more dodge %. That's not a big surprise, though with all the stat multipliers it's less than you might think. Even if the multipliers don't take into account, it's about 3.2% dodge.
However, that's not all that agility gives you. In this specific case, you're also gaining 362 armor and 2.2% crit. Is .6% dodge worth 362 armor and extra crit?
According to Rawr, nope. Even removing all threat value, the Idol of the Corruptor wins over the Idol of Terror by about 16%. If you factor in threat it improves even more. This is fairly intuitive, especially given what we know about fights in Ulduar.
Now, there's a wrinkle to this. Early in 3.2, it was possible to swap idols and have both procs up simultaneously. Later on in the PTR they added this confusing bit of verbiage:
Items with Triggered Effects: These items generally have cooldowns on how often they can be triggered. Those cooldowns are now triggered each time the item is equipped (example: A trinket has a 45-second cooldown on an effect triggered by player attacks; when a player equips that item, the effect will be unable to be triggered for the first 45 seconds it is worn).The problem with this is that as far as I can tell, neither idol has an internal cooldown of any sort. Have they added one since? If they haven't and if this doesn't apply to things in a generic way (ie, giving everything a 5-second delay on equip regardless of ICD), this trick will still work. If you macro all your attacks to something like this:
/equip Idol of the Corruptor
/equip Idol of Mutilation
You should be able to get both procs up all the time. Note that this is not tested on live, so don't take it as gospel; while I suspect that it does work, I can see it going the other way. If so, one of the better things you can do when threat isn't an issue is to use those macros - you'll gain 2-3% more dodge just from doing that.
If anyone else on the PTR has the emblems to test this out, I'd appreciate it; mine were wiped along with all the emblem gear I had bought.