In the old days, people claimed that if you edited "Tornado Slow Aura" instead of "Devotion Aura" that then you could have 3 different abilities made from "Tornado Slow Aura" but where they were set to slow by 0% (no slow) and the buffs would show for all 3 of those abilities, even though if you had 3 devotion auras on a unit then it would combine to one buff icon for the different abilities, effectively hiding the custom icon of your ability.
The way that these systems interact changed on Patch 1.30 and beyond. So it's really only if you are on some older patch like 1.26 or 1.27 that I would suggest you to use Tornado Slow Aura instead of Devotion Aura.