Hi Gunshy,
Just in case you haven’t read it yet the first place I would start your research is the mattress shopping tutorial here (which has a longer and a shorter version) which has all the basic information, steps, and guidelines that can help you make the best possible choices … and more importantly help you know how and why to avoid the worst ones (including the major brand mattresses like Sealy or any mattress that has too much lower quality or unknown materials inside it).
While there is a great deal of information in the tutorial … two of the most important links are post #2 here about the different ways to choose the most suitable mattress (either locally or online) that can help you identify and minimize the risks involved with each of them and post #13 here about the most important parts of the “value” of a mattress purchase that can help you make more meaningful comparisons between mattresses.
Only you can feel what you feel on a mattress and the most reliable way to know which of your choices are the most suitable “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) would be based on your own careful testing (hopefully using the testing guidelines in the tutorial) or your personal experience on a mattress.
Outside of PPP … a mattress is only as good as its construction and the quality and durability of the materials inside it (regardless of the name of the manufacturer on the label) and without knowing the specifics of all the layers and components in a mattress (see this page) it’s not possible to make any meaningful comments about the quality or durability of the materials or the mattress as a whole or identify any potential weak links in a mattress.
If you can find out the specifics of the materials and components of any of the mattresses you are considering then you can either compare them to the guidelines here or if you post them on the forum I’d certainly be happy to make some comments about the quality/durability of the materials inside it.
Phoenix