Okay so,
Cafrisun set works differently than Arde Dagger.
Cafrisun set adds additional elemental damage.
Arde Dagger adds elemental damage.
The names may sound a bit confusing and similar, but they’re very different. Additional elemental damage refers to an additional damage line in normal basic attacks (such as the one you see from getting buffed with Sacrament). Plain elemental damage applies to each hit of any skill or attack that’s not a fake multi-hit.
In your situation, Arde Dagger will give (around) +150 damage for each hit of Cure or each hit of Heal. In short, it’s about the same as if you had ~70 extra INT.
For Cure and Heal, more magic attack will work (almost) the same as more elemental damage. The only difference here is that magic attack will be Holy property (due to Cure’s innate Holy element) while other elemental damage will retain their respective element.
EDIT: Not sure if that was your question now, maybe I misunderstood it?