Hi cari7440,
You can read about some of the many factors that affect the sleeping temperature of a mattress in post #2 here and as you mentioned memory foam and the softness of a mattress are two of the more common factors. Because latex is a more breathable foam and you won’t sink in quite as far, my guess is that it would likely be cooler for most people (but this would depend on some of the other factors as well).
The Blue Essence has 4" of memory foam on top so it wouldn’t likely be as “cool” as the Blue Heaven for most people.
I happen to like the “feel” of 2" or so of latex over memory foam because it provides a more resilient surface and you don’t sink into the memory foam as much but of course this is personal preference and and each person can have very different needs and preferences in terms of which mattress is the most suitable for them. Both of the Therpedic mattresses you are looking at use good quality materials and have no obvious weak links in the mattress.
Stress-O-Pedic is a mid size independent manufacturer (which generally means “better than average” value) but it would be important to know the layers of any mattress that you are considering and they may not be as transparent about their materials inside as the Therapedic mattresses you are looking at. Sometimes you will find that what someone calls a “latex mattress” only has a few inches of latex in it and knowing the layering is one of the most important parts of making a good buying decision and making meaningful comparisons between mattresses. Without this you would be making a completely blind purchase if you were to buy it and it wouldn’t be possible to make any meaningful comments about it.
It’s the job of every good retailer to provide you with this information about every mattress they sell.
Phoenix