Buyers perceive the box spring to be better and generally it is. That is because foundations usually appear in lower end products and have minimal insulation. A foundation that is properly insulated with polyurethane foam is as good as a box spring and costs just as much. The function of the box spring is to support the mattress and facilitate its desired comfort. This function can be achieved with steel, wood, or a combination.
Manufacturers can jump the coil count by using a mattress unit as a box spring.
If both the mattress and box spring have the same units, the coil count is doubled. A properly designed sleep set can utilize a mattress unit as a box spring, but the set will be softer as a rule and the price will be higher. However, the same softness can be achieved by using a conventional box spring at less cost and with superior reliability. The most important considerations are one's height and the number of people sharing the set. If you have the room, buy at least a queen size set.