Actually armor ratings dont act as it written. For example, I having 40 magical defensive ratng as show in my character sheets. When I use a ring which add let say 4 magical armor rating, my total magical defensive rating wont increase to 44, it might just be 42.10 or any other figure. So I can only say, pick the 1 with highest rating you can get, according to which armor type rating you need most.
As for rating type such as rare, heroic and etc, its will reflect on how many extra stats that the armor can come with. Below are list from lower rankings to higher
Uncommon: white, 1 stats no enchantment slot
Rare: blue, come with 1 stats usually, 1 enchantment slot
Magical: Green, 2 extra stats usually, 1 enchantment slot
Heroic: dark green, 3 stats usually, 1 enchantment slot
Legendary: gold or yellow, 3-4 stats usually, 1 enchantment slot
All above can be bought through merchant or gain through chest, which all come with 1 enchantment slots only. And the extra stats might be differ from listed above. Take example for legendary, where usually you can find 3-4 stats on it usually. But sometimes it might just offer 2 stats, with both at very high value. Example +15 melee armor rating.
Only unique gear, (red) has more than 1 enchantment slot, which is 3, and some gear which drop from particular boss or mind read, usually come with white name like uncommon gear, will have 2 enchantment slot. There are rare or magical that have 2 enchantment slot as well, but you cant get them through merchant, usually just quest reward.
