The cinnamon mostly used in Mexican cuisine is called Ceylon, or Canela in Spanish, and it is also known as true cinnamon. It is quite different from Cassia, which is mostly found in US stores. However, as time moves on, one can find true cinnamon in an increasing number of stores here.
The differences are in the looks but also in the flavor and aroma. Ceylon, or true cinnamon (pictured on the right side of the photo), has a milder but sweeter taste. It crumbles much easier, has a lighter color and each stick looks different from the rest. It also has more fragrance than Cassia. Cassia (pictured on the left side of the photo) sticks are much more uniform, so one stick looks very much like the next. It has a darker color, it is much harder, thus very difficult to tear or crumble. While it is less sweet than Ceylon, it has a much stronger, and even a bit harsh, flavor.