Hi DarkJojo,
Unfortunately, there is no universal scale of plushness, and even then there are different “qualities” of plushness that different individuals can interpret differently as well. These mattresses use different componentry, so the only way to tell would be your own actual testing when trying out the product. The upper double layers of latex in the BME would certainly have a more buoyant feel as compared to the Casper, but I wouldn’t be able to tell you if you would think their soft version was softer than the Casper you tried.
I’m guessing your “box spring” isn’t truly a coil box spring, but more than likely a torsion modular system, which was what was commonly used in the 80s. Just realize that even a slight bit of flexion in the box/foundation will impact the comfort of the mattress.
As an aside, if you’re on a more restricted budget then post #4 here and the posts it links to also include many of the better lower budget online options I’m aware of as well.
Phoenix