Cutting Words is almost always better, just by the numbers.
This table gives the probability of meeting a given DC with a straight roll compared to advantage minus a Bardic Inspiration die:
DC |
Straight Roll |
Advantage-d12 |
Advantage-d10 |
Advantage-d8 |
Advantage-d6 |
10 |
0.55 |
0.39875 |
0.47375 |
0.54375 |
0.60875 |
11 |
0.5 |
0.31875 |
0.39875 |
0.47375 |
0.54375 |
12 |
0.45 |
0.23375 |
0.31875 |
0.39875 |
0.47375 |
13 |
0.4 |
0.14375 |
0.23375 |
0.31875 |
0.39875 |
14 |
0.35 |
0.04875 |
0.14375 |
0.23375 |
0.31875 |
15 |
0.3 |
0 |
0.04875 |
0.14375 |
0.23375 |
16 |
0.25 |
0 |
0 |
0.04875 |
0.14375 |
17 |
0.2 |
0 |
0 |
0 |
0.04875 |
18 |
0.15 |
0 |
0 |
0 |
0 |
19 |
0.1 |
0 |
0 |
0 |
0 |
20 |
0.05 |
0 |
0 |
0 |
0 |
As you can see, Cutting Words with d10 and d12 is always better, and the d6 and d8 can be better for higher DCs.
For some intuition here, we can consider that advantage is at most equal to +5 to the roll, for DCs around 10 or 11:
DC |
Advantage |
+5 |
+d12 |
+d10 |
+d8 |
+d6 |
10 |
0.7975 |
0.8 |
0.875 |
0.825 |
0.775 |
0.725 |
11 |
0.75 |
0.75 |
0.825 |
0.775 |
0.725 |
0.675 |
12 |
0.6975 |
0.7 |
0.775 |
0.725 |
0.675 |
0.625 |
13 |
0.64 |
0.65 |
0.725 |
0.675 |
0.625 |
0.575 |
14 |
0.5775 |
0.6 |
0.675 |
0.625 |
0.575 |
0.525 |
15 |
0.51 |
0.55 |
0.625 |
0.575 |
0.525 |
0.475 |
16 |
0.4375 |
0.5 |
0.575 |
0.525 |
0.475 |
0.425 |
17 |
0.36 |
0.45 |
0.525 |
0.475 |
0.425 |
0.375 |
18 |
0.2775 |
0.4 |
0.475 |
0.425 |
0.375 |
0.325 |
19 |
0.19 |
0.35 |
0.425 |
0.375 |
0.325 |
0.275 |
20 |
0.0975 |
0.3 |
0.375 |
0.325 |
0.275 |
0.225 |
As you can see, for lower DCs, advantage is roughly equivalent to +5 to the roll. For higher DCs, +5 to the roll is a much more potent effect. Thus, in every case, Cutting Words, which averages -6.5 with the d12, is going to be much more potent than canceling advantage.
Cutting Words can be used after you know the result.
Restore Balance says:
When a creature you can see within 60 feet of you is about to roll a d20 with advantage or disadvantage, you can use your reaction to prevent the roll from being affected by advantage and disadvantage.
However, Cutting Words says:
You can choose to use this feature after the creature makes its roll
With Cutting Words, you can wait till you know the roll result to decide if you want to use it.