Hard and soft pillows provide unique benefits to different sleepers and it’s difficult to decide which is objectively better. Instead, choosing the right pillow firmness primarily depends on your sleeping position:
Pillows for side sleepers should be firm to keep their head properly supported and prevent the pillow from flattening
Stomach sleepers need softer pillows so their heads stay in line with their spines
Back sleepers generally prefer medium or medium-firm pillows to provide enough pressure and cushioning for their head and necks