Hi luv2sleep,
1.2 lb polyfoam is a “cheap” and very low quality material and the thickness of any low quality materials in the top layer will make a significant difference in its durability.
Foam softens and breaks down primarily in two ways. One is loss of height. If you have a 2" layer of low quality foam and it loses 25% of its height then you would have an impression of .5" which wouldn’t be that significant but if a 6" layer of the same material loses 25% of its height then you would have an impression of 1.5" which would have a much bigger effect.
Even more significant though is the loss of ILD or firmness. If you have a relatively thin layer that loses 25% of its firmness under the heavier parts of your body then your heavier parts (typically the pelvis) would sink in a little more deeply but it probably wouldn’t be enough to cause significant issues. On the other hand … if a thicker layer of low quality material loses 25% of its firmness (and polyfoam of such low quality will probably lose even more) … then your hips/pelvis could sink in much too deeply and you would likely be sleeping out of alignment which can lead to back ache and discomfort. Lower quality materials will both soften and impress much more quickly than higher quality materials and the more low quality materials there are in the top 5" or 6" or so of a mattress (a mattress will usually soften and break down from the top down) … then the bigger an issue both loss of softness and loss of height will be.
An even bigger issue is that the loss of comfort or support that goes with foam softening and the breakdown or cheap materials will lead to the need to replace the mattress but isn’t covered by a warranty which usually requires an actual visible impression (loss of height) of 1.5" or sometimes more before it is considered to be a “defect” (see post #174 here about mattress warranties). The issue of foam softening and “virtual impressions” along with warranty exclusions are the reason that most mattress warranties are mostly about marketing.
In addition to all of this … a two sided mattress that is flipped and rotated every 2 weeks or so for the first few months and seasonally after that will give the foam layers on the bottom a chance to rest and recover some of its height and firmness and it will wear and soften much more evenly (see post #3 here about two sided mattresses).
I personally wouldn’t consider a mattress that used 6" of cheap foam … even for a cheap mattress … unless it was only meant for temporary use or for a guest bedroom where it wasn’t used on a regular basis.
Of course each person has their own personal value equation and criteria that may be most important to them but at least if you decide to buy the mattress you are considering then you will know ahead of time that it’s very likely that you will be replacing it much too quickly and that the warranty on the mattress isn’t likely to make any difference.
Outside of PPP … a mattress is only as good as its construction and the materials inside it regardless of the name of the manufacturer. While neither one uses great quality materials and you would likely have much better options available then either of them … at least the PP could be “somewhat” justified by a very low budget while the KK uses materials that would be very low quality even for the budget range you are in.
Phoenix