The quicker things die, the less they hit you.

Awareness prevents crits on auto attacks if they even happen at all in the time Awareness is up. So Awareness is a preventative measure used when a crit auto could be life threatening. Normally, bosses crit very little. The only time Awareness really shines in non-crit heavy fights is when you're out of tank stance for a short period and want to keep the auto attacks you're taking from the boss smoothed out, or if you're tanking an add that mainly just auto attacks you for a short time. On the other hand, the quicker that add dies the less time it has to actually crit you.

Both are kinda not that great to begin with, it just really depends on how afraid of crits you are. If crits aren't going to kill you or cost your healer's GCDs they could be spending DPSing, then Mercy Stroke is best. But it's really just splitting hairs, both are fine and both have situations where you'll want one over the other.