Shield vs Spirit Armour Spells

[wotan06]

I have a doubt here, please correct me.
It seems to me that the level 1 spell Shield is far better than the level 4 spell Spirit Armour, but that's not logic; how a level 1 spell would be better than a 4th one?

Here's my analisys:

-Both increase AC in the -armour- type (no deflection, generic, etc.) but Shield gives one more (+7 vs +6)
-Both give +3 reflex saving throws (though Shield only vs area?)
-Shield gives immunity to magic missiles.
-Spirit Armour deals damage when worn off.
-Shield is level 1 and Spirit Armour is 4.
-Both have the same duration

So???? Where are the good things of Spirit Armour over Shield ???

[Klorox]

I believe they're from different schools of magic, which means certain specialists won't be able to cast one of the spells. Shield is also limited to the caster, while Spirit Armor can be cast onto other party members.

[wotan06]

True. Diferent schools and target.
But then for my sorcerer-solo project Shield is the way to go over Spirit Armour