Hi carole,
If memory serves me right … the Aruba had a polyfoam core with some very soft thick latex layers on top and a dacron fiber layer. You can see the results of my unsuccessful attempt to find out the layering information in post #42 here.
The term is what I call a “differential” construction because of the difference between the softness of the comfort layers and the firmness of the support layers. In the case of the Ultimate Dreams… it has a firmer polyfoam support layer but you can choose the top layer to be any firmness level you want from 19 ILD and higher. This 3" layer along with the 1.5" quilting would provide a fairly “deep” (4.5") and soft comfort layer. If you go too much thicker or softer (like with the Aruba) you would once again be risking ending up out of alignment after a shorter period of time no matter what the material you were using (depending again on your body type and sleeping positions). Even latex softens over time (just not as quickly as lower quality materials) and if the comfort layers are too thick and soft then even a smaller amount of softening can put you “over the edge” of what you need for comfort and/or support. It may also help somewhat to rotate your mattress end for end on a fairly regular basis (say once a season).
They are very good at helping their customers make the most appropriate choices and of course they would know more about which of the options they offer may be most suitable for your needs and preferences based on the average needs and preferences of others with a similar height and weight, body type, sleeping positions, and preferences. You will need to provide them with all this information of course to help them help you better and information about the types of mattresses you have liked in the past and any results of any local testing you have done may also help them help you more effectively. The best suggestions always come from the manufacturers themselves who know the most about all the different materials and components they offer and how they interact with different types of people.
There are some general layering suggestions in the overviews in the mattresses section of the site and in some of the more detailed pages that are in each section about different body types, different sleeping positions, and how different types of construction (which it seems you’ve read) will affect both of these but again these are more “generic” suggestions rather than specific to any individual because there are too many variables to use a “formula” to choose your ideal mattress outside of using “averages”. For more specific suggestions it’s always best to have a more detailed conversation and work with a manufacturer or merchant directly.
If after a conversation with Chuck you are still undecided between two options … I would go with the firmer of your choices because it’s much easier to soften up a mattress that is too firm than firm up a mattress that is too soft. I wouldn’t “design in” a topper because hopefully you won’t need one and I would keep things as simple as possible but it’s always an option if you need to do some fine tuning as long as the choice you made isn’t too soft for your body type and sleeping positions.
They offer the members of the forum here a “bonus” for their Amazon site which is their shredded latex pillow and a 5% discount fot the mattresses on their Brooklyn Bedding site. I’ve never asked them if they could make sizes that are different from their standard sizing and if you do it would be great if you posted here to let us know what they said. It would be great if they did for those who wanted it and my guess (and this is only a guess) is that if they offered this that they would probably charge you for the next size up and then cut it down to size.
EDIT: To receive the forum discount on their Brooklyn Bedding line make sure you either call, email, or use their chat to get the discount information before your purchase.
I’m looking forward to hearing about what you end up with 
Phoenix