As a doctor and sleep expert, I'm often asked what's the best type of bed: hard, soft, memory foam, futon, etc. There's actually no one right answer. I think we overrate the idea that any particular kind of mattress is better for your back. Is a hard or soft or medium mattress most comfortable for you? Then there's your answer.
But I do urge you to consider investing in an organic mattress and organic bedding. This won't do much for your comfort—but it will help your overall health. Conventional mattresses are loaded with flame retardants, which are problematic chemicals that can be carcinogenic and endocrine disrupting (meaning they screw up your hormones). I think it's an outrage that our bed, supposedly a sanctuary for rest and rejuvenation, is loaded with toxic chemicals.