Hi sully64,
You are in a somewhat difficult position if you need to exchange a mattress and are limited to the mattresses that are sold at the retailer you are dealing with (unless they sell some good quality/value mattresses which many mainstream stores don’t) and in this case I would follow the suggestions in post #2 here and the posts it links to.
Hopefully the store you are dealing with sells some mattresses where you can confirm the quality of the materials inside it so you can make a more informed choice but if not then the mattress/topper approach that minimizes the use of unknown or lower quality materials may be the best approach.
I would try to come as close as possible to the foam quality guidelines here (these are also linked in the tutorial) whether you are buying a new mattress or exchanging for a different mattress during a comfort exchange period. I would try to avoid any one sided mattress that has more than “about an inch or so” of polyfoam lower than about 1.8 lb density or memory foam that is less than about 4 lb density. Once you are at about 2" or more of lower quality materials in the upper layers of a mattress then the risk of foam softening, sagging, or the breakdown of the materials and the premature loss of comfort and support (which isn’t covered by warranty) becomes much higher. Warranties have little to nothing to do with the useful life of a mattress or when you will need to replace it (see post #174 here).
Careful and objective testing using the testing guidelines in the tutorial post is the most effective way to assess which mattress is best for you in terms of PPP. The Sealy warranty information here specifically says that floor models are excluded from warranty coverage (read under the "what is NOT COVERED section).
Phoenix