When deciding whether you need a firm or soft mattress, look at your hips:
Your body type dictates the type of support you need.
If your hips are wider than your waist, a softer mattress can accommodate the width of your pelvis and allow your spine to remain neutral, as shown [above]. If your hips and waist are in a relatively straight line, a more rigid surface offers better support.
Choosing a mattress is a big deal, not just because it's expensive and often difficult to comparison shop, but also because your mattress can greatly affect the quality of your sleep and health.
Check out Cleveland Clinic's article for more sleep posture tips.
Dreaming of a Good Night's Sleep [Cleveland Clinic]