Mattresses come in different firmness options ranging from very hard to pillow like soft. Firmer does not necessarily mean better in terms of a good mattress. If you sleep on your stomach however you probably will prefer a firmer mattress so that your spine is better supported. People who sleep on their sides tend to like softer mattresses because it causes less stress to their pressure points. If you generally sleep on your back you will probably be able to go either way in terms of firmness. In the end it really comes down to what feels good to you. When shopping for a mattress test different ones out and find the one that suits you best.
Regardless of the bed you pick from Twin to California King, finding the right mattress is the most important part of the decision.