Hi woodiesmom,
Post #2 here outlines the approach I would take when you are exchanging a mattress where your options are limited.
The tutorial post here would also be helpful and has the basic information, steps, and guidelines that that can help you make the best choice (and avoid the worst ones) when you are buying a new mattress (or exchanging for one).
Some of the medium density memory foams have a less “stuck in the mud” feeling and a little faster response to them than the higher density memory foams so the Tempurpedic Breeze would make a reasonable choice from a quality perspective although it is not particularly good value compared to other mattresses that use the same or similar quality materials. If you are in a higher weight range (more than about 200 lbs or so) I would be cautious about using 4 lb memory foam because it is not as durable as higher density memory foam.
OMI uses very good quality materials (mostly talalay latex) but as you mentioned they are also quite costly compared to other mattresses that are made of the same or similar materials.
The two most important parts of durability are making sure you know the quality of the materials in the upper layers of your sleeping system (either a mattress by itself of a mattress/topper combination) so that lower quality materials don’t lead to premature foam softening and the loss or comfort or support and to make sure that you don’t choose a mattress that is “on the edge” of being too soft for you initially in terms of PPP and even a even a small amount of “normal” foam softening will put you “over the edge” of the softness range that is suitable for you and lead to the loss of comfort and/or support too quickly.
The better options or possibilities I’m aware of in your area are listed in post #4 here and in post #7 here (which has more detailed descriptions of some of them) but since you are limited to an exchange from the store you are working with this may not be particularly useful to you.
You can see my thoughts about adjustable beds in post #3 here and in the adjustable bed thread that it links to. I know that we like like ours although it’s a preference and not a necessity in our case 
Phoenix