http://wiki.bluegartr.com/bg/Category:Ninjutsu#Elemental_Ninjutsu
On Ni spells, Skill gear is not helping you at all.
On San spells, it will help you until you break 475 skill, but +2 Skill < +1 MAB in most situations.
This is because you should start at 415 skill, which is already a 70% boost. You will generally have less than 70 MAB (outside Abyssea), so it is more efficient to add 1 MAB than 2 skill.
1.71/1.7 < 1.31/1.3, etc.