Real or True Cinnamon is sweet and delicate where as Cinnamon Cassia is strong to peppery Color: Real Cinnamon is a tan color, whereas Cinnamon Cassia is a reddish brown to dark brown. Look: Cinnamon Cassia bark is thicker because its outer layer isn’t stripped off. For that reason, Cassia sticks curl inward from both sides toward the center as they dry. Real Cinnamon sticks curl from one side only and roll up like a newspaper as shown above. Feel: The surface of Cinnamon Cassia is rough and uneven, whereas Real Ceylon Cinnamon bark is smooth.
Real Ceylon Cinnamon is perfect in sweet and subtle dishes that require a delicate flavor.
It is important to own some of both. What they call cinnamon I call Mexican cinnamon (though it is imported from Sri Lanka) and what they call cassia is usually called cinnamon or Asian cinnamon. Sichuan recipes often call for cassia, but then "ordinary cinnamon" will do, you don't want Mexican (Sri Lankan) cinnamon.
I found this statement useful:
First picture [at the above link] shows the soft Real Ceylon Cinnamon sticks. These sticks are very soft and one can see the bristles. Second picture shows the hard Cassia sticks that are reddish brown in color and have single CURL that closes inward. One can easily grind the Ceylon Cinnamon in an electric grinder. You may burn the grinder if tried on Cassia !.