First: a mattress alone does not exist
A mattress only works together with its base. The same unit on a slatted base, on a steel mesh or on a rigid box gives three different sensations. If someone offers you a mattress without asking what it will sit on, they are selling an object, not a system.
Second: your sleeping position matters more than your weight
Side sleepers need the shoulder and hip to sink, or the spine bends. Back sleepers need continuous lumbar support. Front sleepers need a firmer surface so they do not arch. A "medium" bed suits everyone and no one.
Third: heat is half the problem
Many night wakings are not about support but about temperature. Natural materials and ventilated bases disperse heat; closed foams store it. If you wake uncovered and sweating, firmness is not your issue.
Fourth: a ten-minute try is worthless
The body settles in twenty to thirty minutes. A real fitting needs time, shoes off and nobody standing beside you. If a shop will not give you that time, the shop is the problem.
Fifth: ask what happens in ten years
A quality bed can be refurbished, re-covered, repaired. Always ask whether the model in front of you is serviceable or disposable once it gives. The answer changes the real cost per night completely.
Sixth: be wary of anyone who carries only one
A seller with a single brand can only tell you that brand is best. It is not bad faith — it is their position. Ask for a direct comparison between different technologies and watch how the answer comes.