Ideally, do both. I try to use three pillows while sleeping, and I prefer to sleep on a hard floor - beds make my back ache. One pillow between knees, one under my lower back so that my hips are pushes forward somewhat, and one for my head off the the side in case I turn on my side. The one under my lower back is oftentimes actually a dog toy that's just the right size and firmness - if dog toy, not directly under spine, but off on the side that would ache otherwise. My upper torso is straight against the floor, which has improved my posture, while my lower back doesn't touch the floor. If I do it right (meaning, I think about it purposefully before sleeping), I wake up with ninja energy as I do a ninja move bouncing into the air. It was difficult to sleep like that at first, but doing it caused my little muscles between ribs to all strengthen, so basically I got a strong back and healed the pain by strategically sleeping on the floor.