Comfort is your first consideration. Once you find a bed that feels good, find out what's inside. New beds should have at least 1/4 inch to 1/2 inch of this foam. The more of the Visco foam in the mattress, the more durable your mattress will be.
Some beds have only this foam, such as the "Trueform" bed manufactured by Sealy. If you select a mattress that is not all foam, look for the coil count. A queen-size mattress should have at least a 600 coil count, and a king-size bed should have at least a 800 coil count. The coils provide more support throughout the bed and will prevent sagging.
Most mattresses last about 10 years. Warranties might be good for a 20-year lifespan. Expect to spend at least $800 for a bed. A lower cost mattress might have to be replaced in less than three or four years.