Best in slot for tanks is generally considered to be the gear which maximizes survivability. The proc heal relic does exactly that. You can go with the activated power relic if you want, but that's not best in slot. It's an off-beat, albeit mostly viable gear choice, but not best in slot. What you are suggesting is analogous to wearing a piece of DPS gear to improve threat, rather than sticking with all tanking mods.
Threat is a non-issue for assassin/shadow tanks. Go with the proc heal. It contributes substantially to overall survivability.
I'm sorry but could you please post numbers that prove that the heal-proc-relic provides better survivability over a +97def relic?