When buying a pillow we usually look at the filling: polyester fibre, down or memory foam? But there is another question to ask first: how do you sleep?
For a side sleeper, there is a considerable gap between the shoulder and the ear. Filling that gap needs a higher, firmer pillow; otherwise the neck sags all night. A back sleeper is comfortable with a medium-height pillow that supports the head gently. A stomach sleeper needs an almost flat, low pillow. Otherwise the neck bends backwards all night.
The filling serves this need. Down moulds to shape but needs regular plumping. There is a small distinction here: "feathers" are mostly the stiffer, quilled feathers, while the fluffy, light part is the down underneath. Polyester fibre is economical and washable, but clumps over time. A simple test: fold the pillow in half. If it does not spring open straight away when you let go, it is time for it to retire.