Down is rated by “fillpower,” which measures the amount of space that an ounce of down takes up. The higher the number, the longer it will stay firm, though down generally makes for a soft pillow that flattens easily. Down color, which ranges from white to dark gray, depends on when it's harvested and the age of the bird, and generally doesn't affect quality.
These soft and pliable pillows are filled with polyester microfibers, giving down’s softness without the allergy concerns. Firmness and loft, the pillow’s height as it lies flat on the bed, vary by brand.
Memory foam is mostly polyurethane foam that is moldable to the shape of your head and slow to bounce back. It’s supportive and tends to be on the flatter side, though it maintains its height well. Memory foam pillows come in contoured or traditional shapes. The foam can be solid or chopped up in pieces.
These hypoallergenic fibers come in a range of heights and densities, but tend toward the soft, and they often flatten or develop hollow spots, though they can be fluffed up again.