Depending on your skills, at some point the Monk's Unarmored Defense (10 + DEX + WIS) can and will exceed the Tortle's Natural Armor of 17. When this happens, do you get the choice of taking the highest of the two, or are you stuck with one or the other?
IE, will Unarmored Defense always overwrite Natural Armor, or will Natural Armor always overwrite Unarmored Defense, or can you choose between the two?