Hi gonzo,
Just in case you haven’t read it yet … the tutorial post here has all the basic information, steps, and guidelines that can help you make the best choices.
Both of these are high quality materials that use different variations of the Dunlop method of manufacturing the latex foam. Brooklyn Bedding uses 100% natural Dunlop latex that is made in a mold and Spindle uses 100% Dunlop latex (if you choose this option) that is made with a continuous pour method on a belt. In terms of quality they are closely comparable although the continuous pour method can produce latex cores that are a little more consistent in terms of ILD. There is more about the different types of latex in post #6 here.
Outside of your own personal testing (which is only possible if you live near either manufacturer) … the most effective way to choose your firmness options would be a more detailed conversation with each manufacturer. Both of them can provide good guidance about the options they provide that would be most suitable for you (see mattress firmness/comfort levels in post #2 here).
Both use good quality covers and you can read a little more about some of the differences between cotton and bamboo and a wool quilted cover and a cover quilted with polyfoam in post #11 here. The main difference in the quilting is in the temperature regulating properties and in the “feel” of the quilting. Wool is a more costly material and more temperature regulating than polyfoam and polyfoam is usually softer than wool but both materials can reduce the surface resilience and change the “feel” of sleeping directly on latex.
Soy based foams (or other "plant based foams) and polyfoam are really the same thing (both are versions of polyfoam) except “soy foam” has replaced a small percentage of one of the petrochemicals used in one of the main ingredients of foam (the polyols) with a plant based derivative. You can read a little more about the difference between them in post #2 here but I would treat them as equivalents.
One of the links in the tutorial post (post #21 here) includes the manufacturers and retailers that are members of this site that sell good quality/value latex mattresses online (including both of the manufacturers you have mentioned) and have a wide range of different options and prices. All of these would make good choices for different reasons depending on the criteria of your personal value equation that are important to you.
When you have eliminated the worst options and are down to final choices that are all between “good and good” and there are no clear winners between them, then post #2 here may be helpful in making the choice that is the best “match” for all the objective, subjective, and intangible parts of a mattress purchase that are most important to you.
Once you reach this point there are really no “mistakes” any more in terms of quality or value and your final choice will really be a matter of your “educated best judgement”.
Phoenix