Hi MJ,
You will generally find that all the mattresses made by the same manufacturer will be in a very similar “value” range based on the cost and quality of the materials inside them.
While it’s always a good idea to check directly with a manufacturer … I believe that the 10" memory foam mattress uses memory foam that is less than 4 lb density. While all the Dreamfoam mattresses are certainly good value in their budget range … I would tend to avoid memory foam under 4 lb density (see post #4 here) unless it was for temporary use or for a guest bedroom or there were no other options available because of budget restrictions.
The Ultimate Dreams 10" gel memory foam mattress uses 4 lb gel memory foam which besides being a thicker layer is also a better quality material and would be inside the quality guidelines I would suggest using. IMO it would definitely be worth the extra cost.
I believe that both mattresses use base foam that is 1.5 lb density but I would check with them about this as well. (NOTE ADDED: the 10" gel memory foam mattress uses 1.8 lb polyfoam in the base layer which is also a good quality material).
A phone call to Dreamfoam would also be the best source of guidance about the firmness level of each of their mattresses and they will be able to talk you through which of their mattresses would be the best option for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences).
Phoenix