Hi vp14,
You can read some of my thoughts about mattress reviews here. Each person can have very different needs and preferences on a mattress depending on many things including their body type and sleeping style so a mattress that is suitable for one person may be completely unsuitable for the next. In addition to this … most reviews don’t say anything about the quality of the materials in a mattress because most people have little idea about what is in their mattress and without knowing the details of what is in a mattress there is no way to make any meaningful quality of value comparisons. Reading reviews and mattress research (that is focused on finding a mattress that is best for you rather than someone else) are two completely different things.
You are in a somewhat challenging position where you need to exchange a mattress and there are few if any good options to exchange for. Post #2 here has links to other threads with suggestions to some of the other forum members here who were in a similar position. The suggestions boil down to either exchanging for a mattress that in addition to providing you with good PPP (Posture and alignment, Pressure relief, and personal preferences) that also have the best possible quality materials especially in the upper layers which are the most subject to early softening and breakdown. If this isn’t possible (most mainstream manufacturers won’t provide the information you need to know the quality of the materials in their mattresses) … then the best option is to exchange for a mattress which is very firm and uses the least possible amount of questionable or lower quality materials in the comfort layers and then add a suitable and higher quality topper as a comfort layer. In this case you would be making your choice based more on the quality of the materials and the design of the mattress (least amount of lower quality materials) rather than how it felt to you because most of how it felt would come from the topper.
If you do go in the direction of choosing a mattress only without a topper … then you will find some guidelines about how to test it more objectively for PPP (and links to more information as well) in post #1 here.
For the most part I also wouldn’t suggest Costco or any of the big box stores as a good source for good quality or value mattresses and you can read more of my thoughts about this in post #4 here as well.
The choice between the Sealy Destiny and the Serta Insight certainly isn’t one I would want to make because in normal circumstances I wouldn’t consider either of them. The Destiny has 2" of 3.7 lb memory foam over another inch of 4 lb memory foam over a low density polyfoam base layer. The Insight uses 2.75" of 4 lb gel memory foam over a low quality polyfoam base. I personally would try to look for a mattress that would make a good base for adding a topper rather than either of these which are both very similar in terms of the quality of the materials they use. If you are only choosing between either of these and planning to use it without a topper even though they are both on the firmer side, then I would choose the one that your careful and objective testing indicated was the best match for your needs and preferences.
Phoenix