Hi winchesterhouse,
The first place I would start your research is the tutorial post here which has all the basic information, steps, and guidelines you will need to make the best possible choice … and know how and why to avoid the worst ones (which would include all the major brand mattresses you are considering).
Outside of testing a mattress for PPP (Posture and alignment, Pressure relief, and Personal preferences), the most important part of choosing a mattress is making sure you know the quality of the construction and the materials inside it. The information you need to make a meaningful assessment of any mattress or make meaningful comparisons with other mattresses are in this article (and I would print this out and take it into the store with you). The guidelines I would use to assess the quality of the materials in a mattress and identify any weak links in the mattress are in post #4 here. There is also a list of the top 15 manufacturers here and which of them I would tend to avoid and which ones may be “possibilities” if you are able to find out the specifics of what is inside them.
You’re also always welcome to post the specifics of any mattress on the forum and I’d be happy to make some comments about the mattress and the materials inside it or help you identify any potential weak links in the mattress.
Phoenix