Hi ngiovas,
You can see some of my thoughts about Sleep Science in post #2 here and the posts it links to. On the face of it their foam densities are good but they are Chinese produced which means that their foam density may not be the polymer density of the foam (fillers increase the density of the foam but can reduce durability) and they aren’t CertiPur certified. Of course they also won’t be able to provide you with good advice about the suitability of their mattresses for your body type and sleeping style because Costco certainly aren’t what I call “mattress people”. You can read a little more about my thoughts of buying from Costco in post #4 here.
Restava on the other hand uses US manufactured foams that are CertiPur certified and are one of the members of this site which means that I believe they compete well with the best in the industry in terms of quality, value, and service. When you talk with them they will also be able to provide you with much better guidance about the suitability of their mattresses in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) based on the averages of their customers that are similar to you. NOTE ADDED: Restava is no longer a member here since a redesign of their mattresses and they may no longer be fully transparent about the materials in their mattresses … see this topic.
Higher density memory foams are of course more durable than lower density memory foam (if the density isn’t from fillers in the memory foam) but once you are past about 6 lbs or so then the increased durability benefits of higher density is less. 5 lb or higher memory foam is a durable material and would certainly be suitable for your higher weight. Once you are above the lower 200 lbs weight range or so then I would be cautious with memory foam that was less than 5 lbs and include it in thinner layers or not at all.
Hope this helps.
Phoenix