Awareness has a chance it could prevent a crit, or a chance it could do absolutely nothing.

Mercy Stroke will always work, 100% of the time, and is especially useful on fights with adds.

There are fights where you'll want Awareness (Shiva EX, T11 Cube) but otherwise it's not that useful. However, these two skills are really at the bottom of the list, so if you do prefer Awareness over Mercy Stroke you're not really changing your effectiveness either way.