Projectile Damage vs. Skill Damage

Beginner question for Math of Exile:

Culling Strike adds a maximum of 38% more Damage to supported Skill.
Slower Projectile Support adds a maximum of 29% more Damage to Projectiles.

I read that Projectile Damage > all ... but which one is better?

(I'am playing a Deadeye Tornado Shot/Barrage Build for 3.5)