Hi curmudgeon,
These are two different mattresses with different options and designs.
The closest comparison between the two would be the Arizona Premium Adjustable Ultra Plush with a 3" comfort layer and the BB Total Latex mattress but they are also different mattresses with different options. AP has 3 different core firmness options and two comfort layer options (although they also have custom options). Brooklyn Bedding has a choice between 100% natural Dunlop or blended Talalay (not just Dunlop) and has more options in both the support core and the comfort layer firmness levels. They also have different exchange policies and options with AP focused more on exchanging support layers and BB focused more on exchanging comfort layers. In other words each would be attractive to different people with different criteria that were important to them. The price difference is also less than you indicated although AP is a lower cost.
Both of these are great quality and value and once you are down to final choices between good and good then the one that is the “best value” to you would depend on how you felt after your conversations with each one and on all the objective, subjective, and intangible criteria that are most important to you. There are no “bad” choices here but only you can decide which one is the best “value” for you.
In practical terms yes. Based on other factors that affect durability one may last longer than another in a specific construction (in either direction) but I would treat them as equals. The support core is also not generally the weak link of a mattress.
A differential or progressive design is only a concept and the two designs have fuzzy edges between them. One is not “better” than another and it’s more important that a mattress is a good match for you in terms of PPP than the type of design it uses. Some mattress designs would be difficult to classify as either one or the other. The main difference between them is that a differential design has a more significant “differential” between the comfort and support layers while a progressive design has smaller or more incremental differences between the layers or use part of one layer to “help” another.
You are certainly in a good position where you have final choices that compete well with the best in the country 
Phoenix