Hi kvinPA,
The quality and durability of both memory foam and polyurethane foam is primarily determined by density rather than the firmness or softness level of the foam. Both low and high density polyurethane foam can be made very firm or soft. Memory foam is always considered soft (which is why it can’t be used as a support layer) although there is some variation between them and the higher density versions will feel firmer because they generally take longer to soften and mold to body heat.
The Serta is using 2.5 lbs memory foam which is the lowest quality and cheapest memory foam you can get. This also means that it is the least durable. Memory foam that is such low density will soften much more quickly than higher density memory foam (in some cases this may only take months). Because it is so thin though … and because lower density memory foam tends to get softer more quickly (it has less memory) … you would be feeling more of the firmer support layer below it which is “soft” (28 ILD) for a support layer but “firm” for a comfort layer. This also means that its softening will have less of an effect on the overall mattress but the base layer will also soften more over time because it is closer to your body and not as isolated by the layers above it. This could be a risk for good alignment unless you are very light. In other words … from a liquidator (or scratch and dent place) that is likely selling mattresses that have already been used and probably don’t carry a manufaturers warranty … this mattress belongs in the price range you are looking at or less and I certainly wouldn’t pay more for it. It is not very good value and would be comparable to some of the cheapest memory foam mattresses on the market.
The Serta also doesn’t list the density (quality/durability) of the base layer but it should be at least 1.8 lbs density to be suitable and it’s likely that it is (probably between 1.8 and 2.0 lbs). Of course in some mattresses in the lowest budget range it may be lower yet (some very cheap mattresses only use 1.5 lbs as a support layer and Ive even seen 1.2 lbs).
The Tempurpedic on the other hand is a much higher quality mattress and uses higher quality foams in both the comfort layers (4.1 and 5.3 lbs density) and in the support layers (2.2 lbs or higher). It is also very much overpriced however compared to other high quality mattresses that use similar quality foams and sell for much less.
The “air core” layer is a convoluted layer which in theory lets the mattress “breathe” and stay cooler but if a convoluted layer has a solid slab of denser foam above it and the convoluted or egg crate compresses … then the effect of this is somewhat mitigated and questionable.
Lower density memory foam (and polyfoam) is generally more breathable, cooler and faster reacting than denser memory foam and is often used as a surface quilting layer because it can improve the breathability of higher density memory foams. In other words it will sleep cooler than denser memory foam. The foam underneath the memory foam is polyfoam which is generally more breathable than memory foam. the good news is that with only 2" of very low density memory foam … this mattress wouldn’t be as warm as others that use thicker and higher density layers. It will also depend on the type of material used in the cover of the mattress.
There is more in post #2 here about the many factors that are involved in the sleeping temperature of a mattress and there’s more in post #9 here about the different types , properties, and densities of different memory foams.
You are looking at both extremes of the memory foam spectrum. From the lowest quality available to the highest priced of all the higher quality memory foam mattresses. I personally wouldn’t consider either of them. These guidelines may also help you find better quality and value.
If you let me know what city you are in I’d be happy to look and see if I know of any better options in your area.
Phoenix