Hi VictoriaS,
As you can see in mattress firmness/comfort levels in post #2 here … I can only speak to the quality and value of a mattress and comfort choices are always best left to your own personal testing on similar mattresses or to more detailed conversations with a retailer or manufacturer who can help you make the best possible choices based on the “averages” of other customers that may be similar to you.
I personally would place a high value for an online choice where there are good options available after a purchase to make exchanges of either the mattress or layers in case you make a comfort choice that is less than “ideal” but each person may have a different risk tolerance and more or less confidence in the suitability of their choices.
The Ultimate Dreams Ultra Plush on Amazon doesn’t have the option to change the layers after a purchase and a refund with reasonable return shipping costs would only be possible with an Amazon fulfilled order (which would involve some higher costs than their regular return policy) which also means that there would be no pillow bonus
The Cotton Camilla and Ultimate Dreams Ultra Plush would be very similar in terms of feel and performance (with the same comfort choice) and both use the same 1.5 lb polyfoam and the same latex comfort layer. The Ultimate Dreams has many firmness options while the Cotton Camilla only has 2 comfort choices but as you mentioned it would give you the ability to exchange the comfort layer if you made the wrong comfort choice. In most cases the support layers are not the weak link of a mattress and the quality of the upper layers (which are the same) and the upper layers would have the biggest effect on durability although different types of support layers can make a difference in the feel and performance of a mattress as well. they could tell you more about how they would compare in terms of feel and performance with the same comfort layer choice.
I would also tend towards choosing firmer rather than softer if two choices seemed roughly equal because you can always add a mattress pad or topper of various types to fine tune a mattress and make it softer but it’s much more difficult to affect the support of a mattress because this can involve removing and replacing comfort layers that are either too thick or too soft or replacing the deeper support layers with a firmer one (which isn’t an option in this case).
If it is a foundation (no springs) then as long as it meets their criteria here and has no sags, dips, or weak areas and the surface still has non flexing even support then it would probably be fine. If you were choosing a mattress which had latex in the lower layer instead of polyfoam then I would also make sure that any slats were no more than 3" apart but this is not the case here because you are looking at latex hybrids not an all latex mattress.
If it is a box spring (with springs) then they don’t recommend it.
Phoenix