Hi ddt972,
This is a polyfoam quilting layer. There is more about quilting layers in post #12 here.
I would be very cautious about using mattress reviews (positive or negative) as a reliable source of mattress research or information and in many cases they can be much more misleading than helpful (see post #13 here). There are also more detailed comments in post #2 here about “sagging reviews”.
There is also more about the most important parts of the “value” of a mattress purchase in post #13 here that can help you make more meaningful comparisons between mattresses but at your weight and budget range this would be among the most durable choices that are available to you.
Phoenix