Hi jason_555,
This is one of the biggest misconceptions that many people face. A mattress is always a combination of surface softness and deeper firmness because there are two types of support that is necessary in a mattress. The deeper support is to “stop” the heavier parts from sinking in too far and tilting the pelvis which in turn allows the spine (or the joints) to bend beyond a natural range. Surface or secondary support on the other hand is meant to fill in the gaps in the inward curves of the spine and provide more gentle support for the natural curves of the spine to help maintain the natural shape or posture of the spine. The upper “comfort” layers also serve several other functions. One is to provide pressure relief by re-distributing weight over a larger surface area to avoid pressure points which closes off the blood flow in the capillaries. They also “allow” the wider lighter shoulders to sink in enough to keep the curves in the upper back and neck in their natural alignment as well.
The misconception is that “support” is always about firmness when it is really about the combination of surface softness and deeper firmness that is “right” for each individual person. This will vary from person to person based on weight, body type, sleeping position, and the physiology of each person. A mattress that has too much or too little of either (firmness or softness) can both result in a spinal alignment outside of its natural range. Movement over the course of the night is also an essential part of healthy sleeping because no matter how good the pressure relief may be … joints are meant to move into new positions to maintain flexibility and prevent stiffness over the course of the night.
The “art and science” of mattress construction is all about using different materials and different layers to provide this combination of what I call PPP (Pressure relief, Posture and alignment, and Personal preferences) that is most suitable and comfortable for each person. Because there are so many variables involved and because every layer and component of a mattress (including the cover and quilting) will affect every other layer and component … it’s not really possible to predict beyond the “averages” how any person will interact with or “feel” on any particular mattress. Some of these generic or “average” guidelines that can serve as a starting point for height and weight are here and for different sleeping positions are here and some tips and tricks here. Different types of construction or layering will also affect these guidelines in different ways as you can see in the “putting the layers together” overview here and in the more detailed pages in the same section.
What this all boils down to is that there are really only two ways to know how well any particular mattress will work for you.
1. Personal testing for PPP on the mattress you are buying. This is by far the most accurate, especially if it is done as objectively as possible (rather than only subjectively testing for “comfort”). It can be particularly accurate if it is done with the help and guidance of someone with the experience and knowledge to understand the importance of pressure relief, spinal alignment, and the many preferences that people may have that will become more important in their long term experience outside of the showroom. Each person’s needs (pressure relief and alignment which are the two basic functions of a mattress) and their many preferences (many of which are listed in post #46 here) can be very different and some of them (like temperature regulation and durability) can only really be “predicted” by knowledge of the differences between different types of mattress constructions and materials. This is also an essential part of why good guidance can be so important in predicting the longer term performance and satisfaction of a mattress outside of the showroom. The tutorial post includes several links to testing guidelines and suggestions that will be helpful.
2. Buying online or on the phone without testing the specific mattress you are buying. This type of choice is riskier than the first because it is based on “averages” and not on your own personal experience on a mattress. While averages can work for people who are “average” for all their needs and preferences … in most cases one or more of these needs and preferences can be outside of any “average” range and these can become more important over time. For example … sleeping temperature on a particular mattress can be fine for most people but some may still sleep hot. In the same way someone who carries more weight than “average” in certain areas or has a “non average” body shape or sensitivities or physiology compared to an “average” person may react very differently on the same mattress from someone else with the same height and weight or general sleeping position. For those who go in this direction though, there are several things that can lower the risk of an online or telephone purchase.
The first of these is to work with a manufacturer who has a good understanding of how all the layers in the specific mattresses they make or sell (not just some of the components like the foam) have interacted with a larger customer base so they can help you make choices out of the specific mattresses and options they have available that have the highest chance of success. This guidance needs to be based on all the specifics of a particular mattress and not on more generic guidelines. Even the ticking/quilting of a mattress or other seemingly minor differences can play a role that is sometimes quite surprising to those who don’t have more detailed experience or knowledge about mattress construction.
The second way to lower the risk of an online purchase is to use local outlets as a “testing ground” for different types of layering and material combinations. The goal here is not to find a mattress to buy (although this may happen as a side effect if you find good value available locally) but to help identify the materials that you prefer and preferences that are most important to you and where you may be “outside of the averages”.
This kind of feedback can help the manufacturer to use their knowledge and experience with different combinations and with their knowledge of their customer base to help you make better choices in those areas where your needs and preferences may be outside of an “average” range. Of course the benefits of this type of testing for use as guidance for an online purchase depends on knowing the details of the layers and components in the mattresses you test (or your testing won’t provide any meaningful information) and on providing the most objective feedback possible about what you felt on different types of mattress. This can help them to help you choose a mattress out of the options they have available that they believe would have the highest odds of success.
Finally the options you have after a purchase to rearrange or exchange individual layers to fine tune the mattress or exchange the mattress itself and/or the return policy and any costs involved may also be an important part of your personal value equation and the “value” of an online mattress purchase if in spite of “best efforts” your choice doesn’t turn out as well as you hoped.
Because I don’t sell mattresses and know every detail of every mattress that is sold by the hundreds of manufacturers that I have linked to (or that are members here) or have a specific database of customers with different body types, sleeping styles, and sensitivities and preferences that have purchased each specific mattress … my “recommendations” are limited to either generic guidelines or to certain specific “adjustments” that may be helpful with a specific mattress where the exact layering is known and a specific set of" symptoms" are connected to testing or sleeping on a certain mattress.
The goal in other words is to “know your needs and preferences” as much as possible and to connect with the “experts” that can “translate” these into recommendations or suggestions that can help you decide which of the specific mattresses they sell may work best for you.
I wanted to supply a more detailed answer to your request for a recommendation based on your general height and weight and sleeping position because it is such a common belief that there is a formula that works or that a mattress can be recommended based on “theory at a distance” with just height, weight, and sleeping position “stats” without taking into account the near infinite variables of each person or the finer details of each different type of mattress construction which of course isn’t the case. There are just too many variables and personal preferences involved. This way I can use this post as a “reference” for others who may want my recommendation for a specific mattress without a specific reference point and set of “symptoms” on a particular model with known layering.
A second reply with some comments about the rest of your post will follow this one 
Phoenix