Side sleepers really need to fill the area between the crown of the shoulder and the side of the head. We like to call this area the ”Comfort Gap”. The broader your shoulders the larger the ‘Comfort Gap’. The softer your mattress the smaller the “Comfort Gap.” Side sleepers typically need either firm pillows or extra firm pillows. You be the judge but remember the “Comfort Gap” distance does change depending on the width of your shoulders and firmness of your bed.
Stomach sleepers typically need to be soft. Many stomach sleepers sleep on top of their pillow, with some portion of their chest compressing the pillow, treating it almost like a body pillow. In this case, a medium pillow may be perfect. However, where you place your pillow will impact what density pillow is right for you. If you primarily place only your head on the pillow then you need to look for a soft density pillow so that you don’t strain your neck muscles.
Back sleepers, how you position your pillow while sleeping on your back can significantly impact which density pillow you should choose. If you sleep with your pillow directly under your neck or down the nape of your neck at the top of your shoulders then you need a firmer pillow. But if you sleep more with the pillow supporting only your head and less of your shoulders then you may need a medium pillow to prevent your chin from being pushed into your chest.