3. Mexican chorizo is typically seasoned with vinegar and chile peppers, while Spanish chorizo is made with garlic and pimenton (Spanish smoked paprika, either sweet or hot), which gives it its deep brick-red color and smoky flavor. (Authentic Spanish, deeply smoky and tangy chorizo is a revelation--but there's also nothing like a spicy, beautifully greasy-in-the-best-way chorizo taco.)
4. In the United States, Spanish chorizo is available in two different forms: fully cooked and dry (to be sliced like salami/pepperoni), and fully cooked and soft (semicured). LaTienda.com sells both versions (in hot and mild varieties).
5. Mexican chorizo and Spanish chorizo impart very different flavors--and behave differently--so they aren't interchangeable in recipes,