In most cases a firm mattress is always better than a soft mattress as the back then is in a more natural position rather than on a soft mattress. A hard surface is always preferable though less comfortable initially for a good night's sleep as it would help in avoiding any problems associated with the back which would cause one to lose considerable sleep later on.
A firm mattress with inner springs would be the best bet for ensuring a good night's sleep one that adjusts itself according to your specific position without sagging. The best positions to sleep would be flat on the back or lying on the side with the knees slightly bent. Pillows can be used to provide support to the head and neck as well as the legs though in some cases people can sleep comfortably and more importantly in the correct posture without using a pillow.
A firm mattress with inner springs would be the best bet for ensuring a good night's sleep one that adjusts itself according to your specific position without sagging. The best positions to sleep would be flat on the back or lying on the side with the knees slightly bent. Pillows can be used to provide support to the head and neck as well as the legs though in some cases people can sleep comfortably and more importantly in the correct posture without using a pillow.