Hi Coneflower,
I don’t know all the details of your mattress (type of innerspring and all the layers and components) but on the face of it you are certainly looking at a high quality mattress. 4 lb polyfoam is a very uncommon material and ultra high density polyfoam (such as Energia or other similar materials) approaches latex in its performance and durability and I would agree that a two sided mattress using it can have similar or in some cases even better durability than a one sided latex mattress (you can see some of the factors involved in the durability of a mattress in post #4 here).
I believe that Gardner mattress would give you accurate information about their materials and they are an 80 year old company that has built a great reputation for building high quality mattresses that use many hand building methods that are missing from most mainstream mattresses that are currently made. You can see an interesting video here about them that will give you a good sense of who they are and what goes into their mattresses.
If your son has carefully and objectively tested the mattress for PPP (Posture and alignment, Pressure relief, and Personal preferences) so you have confidence that the mattress is suitable for his needs and preferences … and you have made some good comparisons with other mattresses … then I certainly wouldn’t hesitate to purchase a mattress from them as long as the purchase was the best match for your personal value equation.
You can see some examples of two sided latex mattresses made by one of our members that have good quality and value here as a value reference. All latex in a suitable construction and firmness level for the person is durable and long lasting but even with latex a two sided mattress will last longer than a one sided mattress that uses the same type and quality of materials. The tradeoff for a two sided mattress is that you can only use relatively thin layers on each side of a support core in the middle or you would be risking alignment and there are more design possibilities in a one sided mattress where the firmer support core is on the bottom and then you can add thicker layers in various thickness and firmness levels on top of it without risking alignment issues when the thicker softer layers are on the bottom.
Once you have eliminated all the worst choices, there are no weak links or low quality materials in any of the mattresses you are considering, and you are looking at all good options … then there are no longer any “better or worse” final choices except based on all the objective, subjective, and intangible parts of a mattress purchase that are most important to you.
Phoenix