Hi lallim,
The difference would be in the specifics of the mattress which would each have different designs, layer thicknesses, layer firmnesses, and a different cover. Your WBB has 4" of 15 ILD in the top layer and then another 2" of 24 ILD below that which are much thicker and softer comfort and transition layers than many people would do well with in terms of PPP. Every layer and component of a mattress will affects the feel and performance of every other layer and the mattress “as a whole”.
There is more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
I certainly think highly of KTT Enterprises but they don’t sell mattresses that have been tested and passed the fire regulations … they just sell mattress components and layers that can be used as a topper or to build a DIY mattress (although you may need to buy a cover separately depending on the type of cover you prefer).
A component mattress certainly has advantages because you can either rearrange or exchange the layers so that you can customize the support or pressure relief of a mattress after a purchase and you can also just replace a single layer down the road instead of replacing the whole mattress if one of the layers softens or breaks down before the others (usually the top layer) or if your needs and preferences change over time.
Like any mattress choice … which option is “best” for you depends entirely on which one would be the best match for you in terms of PPP and all the other parts of your personal value equation that are most important to you.
Phoenix