Hi DHCC,
Both mattresses are made from foam materials (which includes latex foam, memory foam, and polyfoam) but of course most types and densities of polyfoam are a lower cost material than most types and densities of memory foam or most types and blends of latex. I certainly wouldn’t call polyfoam a waste though since you couldn’t sleep on just a comfort layer by itself and the polyfoam support core provides the deep support that is needed under the softer latex comfort layers. There is more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
There are many people that can’t afford an all latex mattress that is both thick enough and has the combination of layering that will provide them with suitable comfort and support and for these people a latex/polyfoam hybrid can provide many of the benefits of sleeping on latex at a much lower cost (although there are many latex/polyfoam hybrids that aren’t particularly good “value” that can sometimes approach the cost of an all latex mattress). There are even some people who may prefer a polyfoam/latex hybrid over an all latex mattress and there are also some all polyfoam mattresses that use high quality versions of polyfoam that many people sleep very well on and would make very durable choices as well.
Polyfoam comes in a very wide range of firmness levels and different densities that range from low quality/density versions which are “junk” that will soften and break down very quickly to very high quality/density versions which are very durable and in some cases are comparable to latex in terms of durability … especially in the deeper support layers of a mattress which generally aren’t the weakest link of a mattress.
A latex/polyfoam hybrid that uses at least 1.8 lb density polyfoam would generally be suitable for someone in your weight range and even for those that are in much higher weight ranges a higher density support core in the range of 2 lb density or higher would be a very durable material as well.
The choice between different types of materials and mattresses is generally a preference and a budget choice (see this article) and any type or combination of materials that are durable enough for the weight range of the person sleeping on it and that are a suitable “match” in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) can make a very suitable and very durable choice for a mattress.
Phoenix