The rite skills have been designed so that both Lemures and Blood Thorns will deal the same damage per cast when upgraded to single target. So pick your favorite farming/bossing combo based on gear, mechanics or flavor.
I don't think I get this one. Does this mean you're supposed to invest into both elements now? And/or keep 2 gear sets, lets say fire dmg set for farming, cold for bossing? I thought it would work like Lethal incision but then I saw the "passive" tag and now I'm scratching my head thinking how to work with this.