elemental dmg vs projectile dmg for lightning arrow

If you had lightning arrow + a few elemental dmg support gems, would it be better to get projectile dmg or elemental damage (assuming they get buffed by the same %)?

If the percentage increases are the same, then the projectile damage increase would be more worthwhile. However, in your case, probably not by much.

This is because it also increases the skills' projectiles' physical and chaos damage, but you seem to be more focused on your elemental damage anyways.