Hi Mike77,
If it was two sided it would generally list the same comfort layers on both sides of the mattress unless each side has a different firmness level in which case in more “practical” terms it would effectively be one sided since most people would only tend to sleep on the side with the firmness level that they prefer for “most” of the time anyway.
All the layers and components of a mattress will compress to a different degree or percentage of their thickness based on their firmness level, thickness, what is above them, and how deep they are inside the mattress. The amount they all compress together will contribute to how much different parts of your body sink in to the mattress and the depth of the pressure relieving cradle which in turn will affect the comfort and pressure relief and the secondary support of the mattress. Softer layers compress more than firmer layers, thicker layers compress more than thinner layers, all layers compress less when there are thicker or firmer layers above them, and upper layers compress more than deeper layers.
The upper foam layers will tend to compress and deflect more than the deeper layers because the upper layers of foam will absorb and disperse some of the pressure forces before they reach the deeper layers (although you are correct that steel springs have less hysteresis than foam layers) so the durability of the layers that you sleep on directly are more important than the layers that are closer to the bottom of a mattress which normally aren’t the weakest link in a mattress.
If you are sleeping on both sides of a mattress because you are flipping it on a regular basis then the durability of the upper layers on both sides would be important but because you would only be sleeping on each side for about 1/2 the time it would be “safe” to use a little bit less durable materials with a two sided mattress that you flip on a regular basis (see post #2 here). This is why a two sided mattress can be a more durable choice than a similar mattress that has the same comfort layers on only one side of the mattress. There is more about the pros and cons of one sided vs two sided mattresses in post #3 here and the posts it links to.
The mattresses that you test in the stores will most likely already be broken in so I would test them “as they are” assuming that the materials and components in a mattress are “durable enough” for your body type.
Phoenix