My overwhelming preference is to use decorticated cardamom seeds which are simply cardamom seeds that have been removed from their pods. These seeds are then ground in a spice grinder or mortar and pestle before using them in the recipe. This gives the very best flavor and prevents you from having to take the seeds out of the pods yourself (which is a bit of pain). Pre-ground cardamom can be substituted but it will not result in as good of a final product.