Bed mattresses are extremely personal decisions and are often the first major purchase of an adult's life. There are several things one has to consider when you find yourself in need of a bed--many that most people don't even realize.
What does your body need to be most comfortable? If you like hard mattresses, that's great. Do you like them because you've actively thought about it, tried others, and made a decision to sleep only on
firmer bed mattresses? If yes, then you're all set. If your answer is in the realm of, "I don't know why; it's just what I've always slept on, " then perhaps you should try something else, just to make sure. Your body might like something you never thought it would!
But make sure you try out mattresses extensively before you even go to the store. Sleep over at a friend's house, if you know they have a different mattress than you. When you go on vacation, think about how you sleep. Lying down for a few moments on a show room floor is awkward and brief; you will not have the time or the right degree of relaxation to fully understand if you like a mattress.
So you want to save some money on your mattress? Great. But remember that all mattresses are expensive--you might want to save up for a costlier one (in between $500 and $5,000) and have the benefit of its comfort and longevity. Cheaper mattresses are cheaper because they are not as well-made. They will sag much more quickly and may cause back problems. Why buy a mattress you'll just have to replace?
Size is the other key issue. Partnerships must decide on the bed together and must both know how much space they require and like. Look at the actual difference in inches between bed sizes. It helps to both know the number of inches AND to see the difference in person.
Make sure you know what you want; is the extra few hundred dollars really worth a few extra inches in mattress width? For some, it's a pointless expenditure. For others, it makes all the difference in the world.