Hi pglover,
The first thing I should clarify is that mattresses.net and Dreamfoam/Brooklyn bedding are completely different companies. They certainly wouldn’t want to be confused with each other 
Both of them are also members of this site which means that I believe they are among the best quality and value in the country.
There are several manufacturers that offer similar types of mattresses and each have different degrees of ability to customize, different prices, wait times, and differences in the cost and process involved in exchanging layers.
Both offer a choice of support cores (Dunlop or Talalay in different firmness levels).
With Dreamfoam the 3" comfort layer can be both customized, split, and exchanged but the base layer is not exchangeable once you have chosen your preference. The comfort layer is usually the one that is the most common exchange or “adjustment” for those that want to fine tune a mattress if this is an option.
With mattresses.net the comfort layer is a choice between a 2" layer and a 3" layer (each with their own ILD) and the 6" support core (either half or both) is the one that can be customized, split, and exchanged. Exchanging the support core of course will also have an effect on pressure relief but not to the same degree as exchanging a comfort layer. It will however allow you to change the support characteristics of the mattress.
The cost and process involved of exchanging a layer is also different between them.
mattresses.net has a money back guarantee (less shipping) and Dreamfoam on Amazon doesn’t (they do on the mattresses on their own site but it would require more expensive truck shipping).
Each person may also prefer the type of latex that is in one or the other (mattresses.net has more options here).
Once you have eliminated any lower value options from your final choices (which you have) and you are comparing some of the best value in the country … then which one is “best” for each person depends on all the objective, subjective, and intangible differences that are most important to each person and part of each person’s personal value equation. At this point nobody else can really say which one is “best” for anyone but themselves
Either way you have some great choices and some “difficult” tradeoffs between them.
Phoenix