Essentially, each filling will feel the same. These pillows will be firm and each pillow gives you a custom fit for your head, neck and shoulders.
Here are some differences though: Buckwheat hulls themselves are larger than the millet hulls. These hulls will rustle somewhat more when you move. Millet Hulls are tiny and they make little sound when you shift about. Millet hulls are not “softer” than buckwheat hulls, but they do feel “softer” because they are tiny, round, and smooth.
All hull pillows create a very nice airflow and remain cool is the summer and warm in the winter. Buckwheat hulls ventilate more easily given their large irregular shape. This will make a difference for those who tend to perspire at night and wake up to a damp pillow every morning.