Well how much mitigation are you really getting from CoH by comparison.
That damage/pierce on stone might be enough to break even with CoH on damage output, though it would still be 5% leech vs 8%, so I doubt you'd exceed it
For warlocks with amp damage its less pronounced, but on other builds piercing the 33-50% dr on monsters means a big chunk of extra dps, even stacking up next to 246% on demons.
but for incoming damage all coh has is some extra defense, 8% dr and (60-62)->75 light resist. And that defense isn't even much (~1380 coh vs ~720 max on perfect roll greyform). And of course, defense doesn't work against so many sources in PvM. If that fade does activate, that merc caps his resists and gets 15% dr and double curse reduction. Its more like the question is 3% life leech / 8% dr vs 1% chance to cast fade, the other affixes are all kind of gravy.