Guardian's shield is underwhelming, and if you change from a spirit to the other, you don't have access to armor and shield proficiency while you don't have the guardian spirit.
Guardian's Shield (Su): The spirit protects you, granting you a +1 armor bonus to AC, or increasing a light or medium armor's bonus by the same amount. You also gain a +1 shield bonus to AC when you use a shield. The armor bonus increases by 1 at 6th, and 11°.
Considering that an heavy armor is 2 or 3 AC points above a medium armor, I don't think this would be a great deal even combined with the Guardian spirit bonus.
And a medium with this spirit will be one point behind a full plate, two if you wore a tower shield.