Both Stearns and Foster and Simmons are of similiar quality. The fact is there are very few manufacturers of the spring component of both beds. That being said, you should always choose a bed that you feel comfortable on for longer than just a few minutes. Laying on each bed for 15 minutes or longer, undisturbed, should help choose. Take your partner or a friend with you, have them observe your posture in your normal sleeping position. Your spine should follow normal curves, if on your back, or allow your hips and shoulders to be at right angles to the suface if you are a side sleeper.

If I am reading your question correctly, your current bed is not that old. You might want to expand your search and consider memory foam, like Tempurpedic, or air technology, like Comfortaire. Both of these beds will foster proper posture, which will equate to long time comfiness. Especially the Comfortaire can change as your body changes, and is built with higher quality foams, generally, than innerspring mattresses. And both of these choices are designed to last at least twice the life span of the spring mattresses you are looking at, and they shouldn't be twice the price, offering a true value. Spring mattresses, especially south of you are taking market share from springs, and fully 85%+ of customers of these technologies experience better sleep. Since we do not purchase beds that often, they at least deserve a look. Tempurpedic is buying a spring company, which kind of shows the direction bedding is going... It is not how hard a bed is, but rather the support it offers.