d8 to d6 OR disadvantage

[JeenLeen]

EDIT: meant to post to Homebrew. I've requested this be moved.

I'm working on a homebrew system, and I'm debating about how a debuff should work.

In this system, you roll a d8, d6, or d4, depending on how good you are with something.

One debuff I can see doing one of two things: temporarily lowering the die size by 1, OR making them roll with disadvantage (as in 5e: roll twice and use the lower result.) I'm having trouble deciding which to allow.

Crunch-wise: how different are these?

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In a broader sense, what is the difference between allowing advantage/disadvantage verses increasing or decreasing the die size?
Looking at things, I currently have a mix between buffs and debuffs, and I can see it being simpler to be consistent.