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When you use a shield and it has an enhancement value, both of these are added to your AC.
Normally when you use the shield spell, it tends to have a better bonus than the shield you are using so it replaces the shield bonus.
What I am unclear about, since the shield enhancement bonus should be a different type than shield, it should be added to your AC regardless of if its the shield or shield spell being used for AC, correct?
No. The enhancement bonus isn't to your AC, it's to the shield's shield bonus. So what your AC gains is still a shield bonus, not an enhancement bonus (PFSRD, Common Terms, Bonus, Shield Bonus; emphasis mine):
A magic shield typically grants an enhancement bonus to the shield's shield bonus
This is reiterated in the definition of “enhancement bonus” (same page):
enhancement bonuses to armor or natural armor effectively increase the armor or natural armor's bonus to AC
And since two shield bonuses don't stack, you get only the bonus from the magic shield or from the spell shield, whichever is greater.
