Hi Pengel,
Welcome … and I’m glad you found us
The first place to start your research is the mattress shopping tutorial here (there is a longer and a shorter version) which has all the basic information, steps, and guidelines that you will need to make the best possible choice … and know how and why to avoid the worst ones.
There have been many changes in the industry in the last 15 - 20 years and it’s certainly true that many if not most of them are for the worse (see post #3 here and post #404 here and post #12 here among many others on the forum that discuss this). The good news is that there are still many smaller manufacturers that are making great quality/value mattresses that use much higher quality and more durable materials than the major brands and they are available in any firmness level you may need in both one and two sided versions.
There is also more about the pros and cons of two sided mattresses in post #3 here and the posts it links to.
I would avoid Mattress Firm and other chain stores and the major brands and focus on retailers and manufacturers that are more knowledgeable and transparent about the materials in their mattresses (see the guidelines here).
The better options and possibilities that I’m aware of in the Denver area (subject to the “value” guidelines here) are listed in post #2 here. You certainly have many good options available in the area.
I don’t make specific suggestions or recommendations for either a mattress, manufacturers, or materials because only you can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved for anyone to know which type of mattress design or materials would be best for you based on specs (either yours or a mattress) or theory at a distance (see mattress firmness/comfort levels in post #2 here). I can certainly help with “how to choose” but the specifics of “what to choose” is always up to each person based on their own unique needs and preferences.There is also more about the different ways to choose a mattress that can help you identify and minimize the risks involved in each of them in post #2 here.
The mattress shopping tutorial also includes several links to lists of the better online options I’m aware of (in the optional online step).
Almost all mattresses have several layers (including innerspring mattresses) and there are many different types and firmness levels of innersprings so your question really isn’t specific enough to be able to provide a meaningful answer. There is more about the different types of innersprings in this article and in post #10 here and there is more about how an innerspring support core compares to a latex support core in post #28 here if that what you wanted to compare. There is also more about different types of comfort and support materials that are used in mattresses in the “mattress support cores” and “mattress comfort layers” sections of the site and the individual pages that are part of both sections.
Phoenix