Hi laraby,
This would be a preference choice but either way I would either focus on higher density memory foam (5 lbs or higher) or firmer latex. Latex would have the edge in terms of durability.
Again this would be a preference choice but regardless regardless of what type of support system you choose or prefer I would make sure that it was durable enough and suitable for your husband’s weight and your body type which in the case of innersprings would mean more steel and/or lower gauge wire (you can read more about the different types of innersprings in this article and in post #10 here). If you use a firmer support core or “base mattress” than you are comfortable with by itself then you can add a topper as a replaceable comfort layer so if it softens more quickly than the rest of the mattress it can be easily replaced without replacing the whole mattress. The “ideal” would be to test them both together so you don’t have to guess which thickness and ILD topper would work best with your mattress. A component mattresses with a zip cover where you can remove and replace each layer individually would also have a similar advantage. A latex support core would likely have the edge in durability over most innersprings although the support core of a mattress isn’t usually the weak link of a mattress.
I wouldn’t focus on ILD alone because it will depend on the design of the mattress as a whole and your own preferences as well. ILD isn’t the only thing that affects the softness of the comfort layers or the mattress. The thickness of the layers and the mattress itself will also play a role along with ILD. The goal is “just enough” thickness and softness in the comfort layers to relieve pressure in all your sleeping positions. As a very rough guideline I would probably look for something with comfort layers in the range of about 32 or so and then add a topper if this wasn’t soft enough for you but again this would depend on the thickness of the layers and on your own personal testing because thicker layers “act” softer than thinner layers (see post #14 here). It will also depend on the firmness of the deeper layers in the mattress as well which will also have some effect on how soft the layers above them feel.
There are also some mattresses that have a “split” layering where each side can be individually customized for a couple with different needs and preferences (see the first part of post #2 here) but PLB doesn’t offer this option.
Every layer and component in a mattress or sleeping system will have some effect on every other layer to different degrees so it can be somewhat misleading to have a preconceived idea about one particular spec (such as ILD) when there are other parts of the mattress design that will also have an effect on how suitable a mattress is for each or both of you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences).
Phoenix