Idol of Terror, no question. +65 agility is what...about +2.5-3% crit (depending on whether you have BoK or not)? Let's say it only procs half the time...that's still better for you personally than +20 crit rating (< +1.0% crit). It doesn't help your group at all, but this is how I look at it:
+65 agility = (appriximately) 4% dodge, 3% crit, and a bit of armor for you in bear.
+20 crit rating = (approximately) 1% for you, and varies in terms of benefit for the rest of the group.
Different classes will benefit differently from +20% crit rating. If I'm not mistaken, that's not even +1% crit for a hunter (if memory serves, +28 crit rating = +1% crit for a hunter). I think (could well be wrong) that rogues and warriors have it even worse.
In short, we're talking a fairly nominal increase to dps classes and here's the kicker: most tank groups are stacked around buffing the tank, not having the tank buff the melee. You've got a maximum of 4 other people that can benefit from the +20 crit rating...it's not worth stacking the melee in the tank group for such a nominal benefit.
If the raid group is significantly overgeared for the encounter, I could see using the Idol of the Raven Goddess for sh*ts and giggles. For challenging or progression content, Idol of Terror is the way to go (imo).
Edited, Aug 12th 2008 9:12am by AureliusSir