d8 to d6 OR disadvantage

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.

A math question!

Here's the difference, in a handy table:

Original Die	Original Avg.	Reduced Avg.	Disad. Avg.
d8	4.5	3.5	3.1875
d6	3.5	2.5	2.527778
d4	2.5	1.5?	1.875

"Original Die" is your starting die: d8, d6, or d4
"Original Avg." is the average of that die.
"Reduced Avg." is the average for the die one step smaller. (I've assumed d4 goes to d3.)
"Disad. Avg." is the average for the disadvantage mechanic with the original die size (the worst of two d8s in the d8 row, for example).

EDIT: As you can see, one method is not consistently better or worse than the other. Instead, the smaller the starting die, the worse a size reduction is, while the larger the die, the worse disadvantage is. This trend should continue. (I.e., if starting with d10, you'd much rather take a step reduction than disadvantage; if starting with d3, you'd much rather take disadvantage than a step reduction.)

Many thanks.

I think I'll stick with advantage/disadvantage. That also means the users need less dice sizes (no d10s) and don't have to emulate lower than d4 with non-dice. (I was actually picturing d4s doing down to d2, a coin-flip, but that seems annoying.)

D2 is easy... roll a d6.. test if you rolled even or uneven

D3...roll d6 1-2 = 1, 3-4 = 2 and 5-6 = 3

D4 = D4