Hi cantsleep,
Natura was purchased by Spring Air / Sommex which later became just Sommex (see post #7 here). They are a large company and I wouldn’t have any concerns about their stability (although in the mattress industry even the largest manufacturers aren’t immune from bankruptcy).
You can see the specs of the Natura Eco Sanctuary here.
All the materials are good quality and there are no obvious weak links in the mattress so from a quality perspective this would be a good choice … especially for those who liked a thicker layer of wool in the cover.
Any new mattress will be a little firmer than a showroom mattress that has broken in and it can take about 30 - 90 days for the materials to settle in, the foams to lose any of their initial “false firmness”, and for the fabrics to stretch and lose any of their initial stiffness.
It depends to some degree on the weight of the wool and on how it is compressed or quilted but with thicker layers of wool you can expect some degree of compression over time (in the range of up to 30%). This is a normal part of how wool performs over time and isn’t a quality or durability issue (unlike polyfoam where visible impressions are a sign of the material softening and/or breaking down). Wool that becomes more compressed will become a little firmer.
I would also be very cautious about the Prana mattresses. You can read more about them in post #3 here and in post #2 here and in post #3 here. A forum search on Prana (you can just click this) will also bring up more information and feedback about them.
If I was choosing between only these and they were otherwise similar in terms of PPP and the parts of my personal value equation that were most important to me then I would lean towards the one that doesn’t have potential weak links in the design (the Natura).
Phoenix