
Cinnamon stick is the dried inner bark of the cinnamon tree, rolled into a quill and used whole as an aromatic spice.
A pinch of ground cinnamon delivers faster and stronger and leaves the drink hazy. Cinnamon syrup gives the flavor and dissolves cleanly, which suits a shaken drink where a stick would do nothing in the time available.
Cassia bark
Thicker, harder, more pungent, and much higher in coumarin than Ceylon cinnamon. Most sticks sold as "cinnamon" are cassia; a recipe written for the delicate Ceylon kind will come out louder than intended.
Ceylon cinnamon
Thin, brittle, layered like a cigar and it breaks in the fingers. More fragrant and less hot, and it gives up its flavor to a syrup faster because the bark is thinner — so it needs less time on the stove.
Sealed and dark, sticks keep for years, far longer than the ground spice, because the aromatic oil stays inside the rolled bark. A stick that no longer smells when you snap it is finished.