>> all foam mattresses deform to some degree
So do spring mattresses, by the way, and aren't they the most common type?
So do spring mattresses, by the way, and aren't they the most common type?
Personally I prefer spring mattresses over foam mattresses. They last much much longer as dangrossman commented, are more reliable and also much firmer. And also you can't leave aside the weather factor because spring mattresses are way more breathable than foam-only mattresses. We, for example, avoid selling foam-only mattresses to people that live on high humidity areas because there is a high chance they will return it with humidity stains.
Normally I would go for a pocket spring mattress ( I think that's the term in English for a mattress with individual sealed and padded springs ) with a decent amount of memory foam on the top. That way you get the firmness and breathability from a spring mattress plus the comfort of a nice soft memory foam layer.
On the firmess you're right, it is very variable. I've been myself sleeping on a foam-only sofa-bed mattress for almost a year and it is firm enough. The issue with latex or foam is that they distort with time. It is unavoidable. Depending on the quality of what you have bought that might be years but I've seen many poorly made mattresses that are returned within months.
It also depends on the weight of the people that is going to sleep as well. My wife would be worried if you were a customer insisting on buying a foam mattress and your weight as a couple were bigger than 180kg for example. Specially if it is a wide mattress as the weight would not be distributed evenly.
(I ended up buying 2 awesome, cheap, non-brand name, made in the US mattresses from Restonic, which are still going strong after 5+ years).