Common cumin is both healthful, tasty, and medicinally useful. It's also nutritious, due to its abundance of vitamin C and vitamin A. Cumin also contains dietary fiber and has stimulating, anti-microbial and anti-fungal properties. Finally, cumin is a great source of iron, an important mineral.
Black cumin is more bitter and may be a bit strong and bitter for American tastes (It's used in specific dishes in northern and western Indian cuisine). But the medicinal qualities alone make it a useful herb, even if you just munch on its seeds or use them to make an herbal tea! Whether you choose common or black cumin, you'll be adding something really special to your herb garden!