Blade skills get a mixture of bonuses from claw mastery based on which blade skill you are using. There is a chart out there that shows exactly which blade skill gets what bonuses from claw mastery.
That’s why you are seeing your cestus perform better than that other weapon even tho the other weapon has slightly higher damage.
There isn’t an exact science to this but if you get a non claw weapon that has significantly more weapon damage then a claw, you would be better off using the other weapon, not claw.
But skills are very important for blade skills. Making using a spirit sword likely better than anything else for low levels