According to the research, side sleepers mostly prefer a medium soft mattress as it relieves the pressure on their shoulders and hips meanwhile it gives support to the spinal alignment also. But if you are suffering from back issues and neck pain then a firmer mattress is more preferable as it helps in hitting the sweet spot of your back. So it is suggested to go with a medium-firm mattress for side sleepers having back pain to help them with their aches and back pain.