Dexers are the only ones that genuinely benefit from 120 bushido, and that can be debated.
This is incorrect, and there is no debating it.
At 100 Bushido, you have a 13% chance at a Critical Strike.
At 120 Bushido, you have a 20% chance at a Critical Strike.
Critical Strike applies to both melee and archery.
At 100 Bushido, it takes 10 steps to reach Perfection.
At 120 Bushido, it takes 9 steps to reach Perfection.
Perfection applies to both melee and archery.
120 Bushido heals more with Confidence than 100 Bushido does.
Confidence applies to both melee and archery.
I wouldn't have GM Bushido on my archer personally. The benefits of having 120 are more than justification for using the skill points.
To answer the OP-
Put it on the dexxer, then the archer later. You get better Parry chance with 120 Bushido than 100, plus everything I listed above, along with a few other bonuses.