Ceylon cinnamon is a pale tan colour, and comes in rolled quills made up of many paper-thin layers of the bark rolled up in one another. It is quite fragile, cinnamon-y but quite sweet. My experience is that many of my customers like to eat it straight. It also comes in quills, but each quill is a single roll of a bark the thickness of heavy cardboard. The cinnamon-y flavour is stronger and almost pungent, and the wood is harder.
Chinese cassia is usually quite dark brown.
It normally comes not in rolled quills but rather as irregular pieces of thick bark, some of which may be partially curved and others flat. It has a very strong cinnamon-y flavour, but little sweetness. Before that time in Europe cinnamon and cassia were used only in medicine and perfumery. In Arabic sources the confusion can catch up with you. So for recipes after the thirteenth century, Ceylon cinnamon would be the choice of cooks if they could afford it.