Hi hiddenspring,
The first place I would start your research is post #1 here which will give you all the basic information, steps, and guidelines that you will need to make the best possible choice … and eliminate the worst ones (which would include Serta mattresses and other major brands along with any manufacturer or retailer that isn’t able to provide you with the specifics of the materials in their mattresses). There is really no way to know the quality or value of a mattress unless you know the specifics of the quality of the materials inside it and without this you would be making a completely blind purchase that can’t be meaningfully compared with other mattresses that may be much better in terms of quality, durability, and value.
As you can see in mattress firmness/comfort levels in post #2 here … I’m happy to help you assess the quality of a mattress as long as you are able to find out the specifics of all the layers and materials (which Serta generally doesn’t provide) but the suitability of a mattress in terms of comfort and support or what I call PPP (Posture and alignment, Pressure relief, and Personal preferences) can only be known for certain through your own personal testing or experience. A mattress that provides you with good PPP may be completely unsuitable for someone else and there is no “formula” that is more effective than your own personal testing or experience.
You can read about the many factors that are involved in the sleeping temperature of a mattress in post #2 here and you can read more about the pros and cons of memory foam in this article but in very general terms memory foam tends to be the least breathable of the different foam types (although there is a wide range inside the memory foam or gel memory foam category). You can also read a little more about the different methods that are used to cool down memory foam in post #6 here for those that tend to sleep warm on memory foam but still prefer its feel and performance.
Memory foam also has a slower response than other types of foam so it is not as resilient and movement friendly as other materials although there is a wide range of different responses between different types and densities of memory foam and the thickness of a memory foam layer or layers and what type of foam or material is underneath it will also play a big role in how the mattress feels and responds to movement and changing positions. Any material can provide good pressure relief and support/alignment in a combination or design that is suitable for you but the choice of material is a matter of personal preference more than being “better or worse” for specific types of people or circumstances.
For most people neither of these are a big issue and are worth the tradeoffs that are involved in all material choices but for some where these are more important parts of their criteria they certainly are. It depends again on your own preferences, tendencies, and sensitivity.
If you do choose memory foam then I would also make sure it meets the criteria in post #10 here so that the chances of being affected by any offgassing is minimized (some people can be more sensitive to this than others).
The key for a weaker back (or any back) is not so much which material you use or prefer but the specific design of the mattress that the material is used in. All materials can provide you with good alignment as part of a design that works well for your specific body type or sleeping positions.
So overall and for some people the properties connected to memory foam would be enough to exclude them from consideration while for others there is nothing else they would rather sleep on. Your own sleeping experiences (whether you tend to sleep warmer than others for example) and knowing your own material preferences through personal testing and more detailed conversations with knowledgeable and experienced manufacturers or retailers about the specifics of their mattresses are the best way to decide which types of materials or mattress designs are best for you. I would tend to avoid thinking in “better/worse” terms and more in terms of which of the pros and cons of each material or mattress choice is most important to you.
Some of the better local options in the Raleigh/Durham area I’m aware of are listed in post #6 here.
Phoenix