If you like the feeling of sinking into your bed as opposed to resting on top of it, then a memory-foam mattress made entirely or mostly of polyurethane foam (aka polyfoam) may suit you best.
That said, you can certainly get a solid night's sleep for less: Take the Nectar Classic Memory Foam Mattress ... and the mattress expanded to full size by the time I went to bed 10 hours later.
If you’re primarily looking for pressure relief and support, latex foam is my top choice for a cooling mattress topper because it offers a similar feel to memory foam but without as much heat ...