I have had back pain since I was a teenager. I would say the memory foam mattresses are the best. But if you can't afford one of those get a really good memory foam mattress topper 2 inches or more thick.
First of all I wouldn't touch a spring mattress, not good for posture ;) I would also stay away from a mattress topper, I would just go straight for the whole thing and have the whole mattress in memory foam (unless you are looking for the cheap opti...
I don't have a brand of mattress to tell you about, but maybe something that will save you a little money. I too have a bad back and found that I can get away with buying a relatively inexpensive mattress and just add a 3-4" memory foam mattress topp...