Hi Stanton,
The first place I would start your research is the tutorial post here which has all the basic information, steps, and guidelines that can help you make the best possible choices … and know how and why to avoid the worst ones.
Outside of how suitable a mattress is for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) the most important part of a mattress purchase is knowing the quality of all the materials inside it so you can identify any weak links and make meaningful comparisons with other mattresses. The information you would need to make an informed choice (and avoid the risks of making a “blind” purchase) is in this article.
I would want to know the density of all the polyfoam and memory foam layers in the mattress (which will almost certainly be a weak link in the mattress) along with the type and blend of the latex. I should also note that the Costco site has two different descriptions of the layers in the mattress and neither one adds up to the thickness of the mattress which indicates that neither one is correct and/or complete.
IMO … you would certainly have better quality/value options available to you.
Phoenix