Hi Mike77,
I would keep in mind that coil gauge by itself wouldn’t be a reliable indicator of the firmness of the innerspring, although if all the other variables in two innersprings are identical such as coil number, coil shape, coil height, number of turns, coil diameter, coil arrangement, type of innerspring (linked or pocket coils), then a lower gauge innerspring with thicker wire will be firmer. The type and thickness of any padding above and below the innerspring and the specifics of the cover will also have a significant effect on how firm a mattress feels as well, and the innerspring is rarely the “weak link” within a mattress. You can read more about innersprings in this article here, and learn about the many variations and differences in the main types of innerspring units.
Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that feels firm for one person can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here) so different people can also have very different opinions on how two mattresses compare in terms of firmness and some people may rate one mattress as being firmer than another and someone else may rate them the other way around. This is all relative and very subjective and is as much an art as a science. In other words … the only reliable way to know whether a mattress will be “firm enough” or “soft enough” for you will be based on your own careful testing or your own personal experience.
The two basic functions of a mattress (support/alignment and comfort) are explained in more detail here. In general, the innerspring unit (or latex core or polyfoam core) provide the deep support within a mattress, and the upper layers address comfort to a greater degree. There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
Whatever comfort materials you choose, always make sure that you find out the information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
If you wish to be able to change layers, you may wish to look for a foam or latex component-style system which allows for layers to be rearranged or changed out.
You can follow some of the guidelines presented in this article, but I would caution that analyzing alignment can only truly be done by a qualified professional, and even then it is rare for someone to have a perfectly “neutral” alignment that would match most anatomy textbooks, and your own personal perceptions will generally be very accurate.
Phoenix