I use Mercy over Awareness.

Though that's largely because I can't be bothered to level GLD up on my character that actually has a DRK.

Mercy Stroke is better for solo content since crits don't really matter there.

Awareness is better for dungeons (smoothing out damage on large packs) and raids (post-tank buster scenario, auto-crit adds, etc). I never had any problem not having Awareness outside of Savage though (where I'd prefer the mitigation over the damage).