Foams used for chair cushions and mattresses come in many different densities.
The difference with high density foam is that it is heavier and more solid and durable. Higher density foam mattresses are particularly good for people who carry extra weight and who like a firm mattress to sit on. The weight of the foam determines the quality. The hardness doesn’t! The higher the density of the mattress, the heavier the mattress will be to move around. Higher density foam mattresses are more durable, last longer, perform better and have less compression’s over time. Higher density foams can be made to be soft or hard. Higher density foam is more expensive, but “you get what you pay for!”
If your mattress is going to be outdoors and subject to the elements, you can also get marine-grade foam which prevents water absorption and mould.
Cheap lower density foam simply doesn’t last the distance and can be very disappointing. The hardness loss in cheap foams is very evident in low density foams. Higher density foams lose 10% hardness over time, whereas cheap lower density foams can lose approximately 40-50% hardness. A lower quality foam can also be made to feel soft or hard. Lower density foams can also be prone to tearing when they age and can get mouldy (see photo above).
It would be very disappointing to buy a lower density foam when having a daybed recovered to find that it gets saggy and the fabric doesn’t sit nicely. The hardness of the foam is simply up to the individual.