The difference is so minor that in the end it doesn't even matter. I find it funny how ppl go apeshit over someone wanting to meld Tenacity, like its this big catastrophe. Yes its less dps (ever so slightly nothing that will change anything) and yes you take less damage, regardless what you may assume that this part doesn't matter it certainly doesn't hurt either when the penalty is not larger than it is.

Personally I'm pentamelding Tenacity for savage progression because A) it helps the healers slightly when you actually have 10% permanent dmg reduction, and B) its dirt cheap to meld so win/win. Once I'm getting savage upgrades I'm not gonna meld Tenacity onto them but for the pentamelded gear, why the heck not. It's not like you will fail the fight over the tank doing 150 less dps than they could with different melds, and to be honest, no-one gives a damn about first clear dps as long as the boss dies. But on the flipside i have survived plenty times where i would have died without it.

This damage first mentality that ppl seem so obsessed over is really all in your heads.