Choose the firmest coil spring mattress available. Firm springs hold up to increased weight better than softer springs. If you are on a budget, get a lesser-quality firm coil spring mattress, but expect it only to last as little as half as long as the warranty suggests. In all cases of coil spring mattresses, the more springs it has and the thicker the spring material, the longer it lasts. Plush, pillow-top mattresses are not going to be the best choice for a heavy person because of the need for support.
Consider a close-packed, high-density foam mattress. These slightly soften over time, especially for someone who is overweight, but the mattresses give a good account of themselves over their lifetimes. The firmest of these types offer superior comfort over a spring mattress, but may not be quite as firm without springs. With no springs to wear out, there are no pressure points on certain areas of your body that may make sleeping uncomfortable.
Choose a modern, adjustable air mattress (not the portable blow-up kind) for the comfort of a foam mattress and the firmness of a spring mattress. Air mattresses are made of heavy duty materials that allow an inner pump to inflate the mattress area. You dial in the firmness you want or make them as soft as you like. On some models, the air bladders are separate between the right and the left side. You may choose a firmness setting that is independent of the other side of the mattress. If you are overweight but your partner is not, this may be a critical advantage for restful sleep.