The first armour, you have two 'free' charms to add before they can be compared on an equal footing.
I ignored the Aura of Guarding bonus on the second armour, since spell books can be bought (that is how I maxed this skill). Kistandalius or Blake in Ars Magicana and Corinna or Mpenzak in Verdistis sell spell books, as does Kroxy in the wastelands.
I sold all my loot before going to the wastelands (29 stacks of 50k gold, plus change), and bought all the charms and spell books Kroxy offered, unless I already had the skill maxed. The imps in the wasteland can drop gold charms, as can others.
