If the servers were up, you could press C and look to the left of your HP in that window to see an icon, hover over it, and see exactly how much AC contributes to damage mitigation. Think of it as the innate toughness of the item you're wearing (like wearing plate armor vs cloth armor in any mmo). Defense and AC both…
If I were to go in on this by buying the $60 pack, and then found myself blown away by the first beta weekend to the point where I could not exist without an armored spider mount, would I be able to upgrade to the $200 pack?