Many people are picking Swordman 3 right now because Double Slash can be copied by shinobi clones.
Barbarian c3 is decent but it can’t compete with the dmg output Moulinet or Double Slash + clones give. Barbarian c3 doppel is not so great atm because it leaves you with very little options for rank 9. Taking swordman 3 at rank 9 is a really big waste tbh. Taking shinobi with barb c3 is also a waste cause you’d have basically no skills for clones to copy.
The builds might change a little once shinobi is able to copy more skills. When Seism becomes clone-able, barb 3 might become much more popular than what it is atm.
You want to max DoV in any Doppel build you make. Not only the skill point investment is better in DoV than in normal doppel skills, it also helps with skills aside from Doppel’s arsenal, such as highlander / barb skills.