Hi fjbpatt,
There are some memory foam designs and materials that will sleep a little cooler than others so depending on where you are in the “oven to iceberg” range, the specific design of the memory foam mattress and the type of memory foam it uses, and on the type of mattress protector and sheets you use, you may find that you will be OK on some memory foam mattresses in terms of sleeping temperature even if you tend to sleep warmer on others. There is more about cooling down memory foam in post #6 here.
Polyfoam tends to be more breathable and cooler than memory foam and polyfoam itself isn’t an issue with durability … only lower quality/density polyfoam (which is what you will typically find in the comfort layers of most major brand mattresses). There are many high quality and durable mattresses that use polyfoam that won’t have durability issues or issues with premature body impressions or sagging. This is why it’s so important to make sure you know the quality of all the materials in any mattress you purchase so you can confirm that there are no “weak links” in the mattress regardless of which type of material or mattress you tend to prefer (see this article and the foam quality/density guidelines it links to).
Phoenix