Hi Rachierad,
I haven’t slept on either of them so I really can’t speak to any differences between the “feel” of the different memory foams or the mattresses as a whole. Feel is very subjective and can be experienced differently by different people. It will also depend on all the mattress layers and not just the memory foam layers. For this you would either need to actually test or sleep on them yourself or talk with each retailer or manufacturer so they could tell you their own thoughts about how each one “feels” and where in the range they may be compared to other memory foams in terms of “feel”, softness, temperature response, recovery rate, or breathability (although I suspect that Aerus would have the advantage here over most other memory foams).
The lower layers will also make a difference in the overall “feel” of the mattress … not just the memory foam itself.
The one advantage of the Rocky Mountain mattress that would be more important to me than it appears to be for you is that you can re-arrange the layers to adjust the feel and performance of the mattress if your actual experience is different from your testing and you could also exchange a layer if that was necessary which is an option not available in the Easy Rest. You can also replace a single layer down the road if your needs and preferences change down the road or if one layer softens or wears out faster than the rest (usually the upper layers) without having to replace the entire mattress. You could also use the 5 lb memory foam on top which would increase the durability of the lower density memory foam below it.
This depends entirely on how you personally define “safe” or “how safe is safe enough” for you. You can see the criteria I usually suggest for memory foam mattresses here (which suggests that either the foam is US made as in the case of Rocky Mountain or CertiPur certified if it isn’t which is the case of the Easy Rest where the foam is poured in China and Rocky Mountain as well). A small percentage of people that are much more sensitive to offgassing or any chemical exposure or who on principle alone would prefer to use more natural materials may avoid memory foam or even polyfoam completely. This is a complex and controversial subject and you can find much more information in post #2 here if you wish to research and explore it.
You’re correct that 4 lb memory foam is not as durable as 5 lb memory foam. The tradeoff is that many people prefer the feel and response of 4 lb memory foam so this would be one of the tradeoffs that were part of each person’s personal value equation along with the other options and benefits that are part of any mattress purchase that include the mattress itself or the retailer or manufacturer you are dealing with. In most cases it won’t be actual impressions you would see but foam softening under the areas of greatest body weight which results in the loss of comfort and/or support faster with lower density foams. the effect of foam softening will depend on the person and where they are in the “range” of pressure relief or support they need and how much softening it would take for them to cross the line into “too much for them”. If you are over 200 lbs or so then I would probably lean more strongly towards the use of higher density foams but again this is personal preference and depends on what is most important to you.
Phoenix