Hi Jarwulf,
The first place I would start your research is the mattress shopping tutorial here which has all the basic information, steps, and guidelines that you will need to make the best possible choices … and know how and why to avoid the worst ones.
Outside of the suitability of a mattress for your body type and sleeping positions in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) … a mattress is only as good as the quality of the materials inside it regardless of the name of the manufacturer or the brand because all manufacturers have access to the same or similar materials. There is more about the most important parts of the “value” of a mattress purchase in post #13 here and I would always make sure you can find out the specifics of what is inside any mattress you are considering so you can identify any weak links in the mattress and make more meaningful comparisons with other mattresses.
If you can find out the information in this article about any mattress you are considering and post it on the forum then I’d be happy to make some comments about the quality and durability of the materials inside it but without this information you would be making a completely blind purchase with a much higher chance of buyers remorse as a result of durability issues much too quickly after a purchase.
Phoenix