Depending on your sleep position, you will have pressure points on different parts of your body. For example, if you sleep on your side, you want a lot of support for your shoulders. But if you are a back sleeper, you want a lot of support in your lumbar region. Different firmnesses will support these specific regions of your body differently.
Many side sleepers prefer a soft mattress, because the plush contours and sinkage support both the hips and shoulders. Side sleepers have an arm that gets tucked underneath them while sleeping, and a mattress that is really firm will cause it to feel crushed, numb, and tingly.
Back sleepers have a wide range of firmness preference because pressure points in this position are more evenly distributed. So take your pick on firmness, but be sure that you have a good support system nonetheless. For those that experience back pain however, you will most likely want a mattress on the medium/firm side.
Stomach sleepers often prefer a medium to firmer mattress, because they need additional support in the middle region. Too plush of a mattress will cause the hips to sink lower than the rest of your body, which can cause back pain. If you like a plush feel and you’re a stomach sleeper, opt for high resiliency foam, so that you get the push back coupled with a soft contouring feel.