Hi mermla,
Unfortunately you are in somewhat of a difficult position when you need to exchange a mattress and there are only very limited options available to you. There are many others that have been in a similar position and post #2 here and some of the other posts and threads it links to has some of the suggestions and ideas that you will find helpful.
Post #1 here and the other posts it links to would also be worth reading and has the basic information, steps, and guidelines that may be helpful. It also has some suggestions for testing mattresses for sleeping posture/alignment and pressure relief and PPP (Posture and alignment. Pressure relief, and your Personal preferences) is one of the most important parts of choosing a suitable mattress along with minimizing lower quality foam or other materials in the comfort layers so that foam softening doesn’t lead to the loss of comfort and support prematurely. Foam softening and the loss of comfort and support is the main reason a mattress needs to be replaced and isn’t covered by warranty (see post #174 here).
These posts should give you the information you need to make the best possible choice in difficult circumstances. Testing mattresses at other stores won’t be particularly helpful because I am assuming that you need to exchange the mattress for another one at the same retailer and that you don’t have a refund available (which would be ideal).
These are not mutually exclusive because there are mattresses that use innersprings for the support layers and also have highly conforming foam on top. The Serta iSeries is an example of this. Any type of comfort layers can be used over any type of support layers. All support layer materials or components (whether they are innersprings, polyfoam, or latex) come in a wide range of firmness and support levels and comfort layers also come in a wide range of thickness and softness levels as well regardless of the type of support system that is used under them.
You also have a fairly large credit available for the exchange which means that you also can exchange for a higher cost mattress and some of the iComfort mattresses and many of the Tempurpedic mattresses use reasonable to good quality materials in the most important comfort layers so the issue with these is more about their relatively poor value and not so much the quality of the materials. Without a refund you are locked in to lower value mattresses anyway though so the goal with an exchange would be to make sure you do good testing to make sure that the replacement mattress is suitable for your specific needs and preferences and that you minimize the amount of lower quality materials in the comfort layers either by knowing the density of the foams or by choosing a mattress that minimizes the use of low quality memory foam or polyfoam and adding your own comfort layer in the form of a topper.
There is some information about the materials in the iComfort line in post #11 here and about the Temurpedic line in post #10 here. In terms of the quality of materials there are some reasonable choices in both
If you let me know which store you are dealing with I can take a look at their website to see if they carry any other mattresses that may be worth considering.
Hope this helps.
Phoenix