The modifier for Siphon Souls, Sear Souls, is I feel no choice at all, you should pretty much always take it.
The dowside is … -10% total damage for the skill. Alright, I guess.
But the upside is … 150% uptime instead of 50%. Oh and the damage type (partially, no aether dot after all) changes to one that most necromancer skills (Harbinger of Souls, Spectral Wrath) and a ton of devotion already supports. Not to mention gear.
So uh, I try to find reasons not to take this modifier, maybe when you’re building a pure vitality character? Even then there is a healthy part of vit decay. 10% damage penalty is not relevant at all I feel, the uptime boost negates it and then some.