Originally Posted by
Reinhardt_Azureheim
Well. Kind of, but there are differences. For a start, this would be a net -1% magic mitigation (0.9 * 0.9 = 0.81 -> 81% damage remaining -> 19% mitigation), not a debilitating decrease but I'd rather keep it stable.
For another, it would then grant mitigation against Unique damage (i.e. not-physical + not-magical). Generally this only really applies to Heavensward and prior (breath attacks often had this, like Ifrit's breath) as Unique damage is mostly attributed to unavoidable mechanic damage.
Final two points would be less spaghetti code for CBU3 devs to work on (damage calculation formula) and it is simply easier to digest when you read it as a player. 20% magic, 10% phys has zero ambiguity over what it does. Plus it is easier to argue for the implementation due to Addle having basically this effect but in debuff form.
TL;DR: both are fine, but I see my suggestion more realistically added - gut feeling.